halkyon/packages/halkyon_base_server/README.md

# halkyon_base

What every Halkyon server has, as a Serverpod module (`halkyon_base_server`, with its
generated client `halkyon_base_client`): signing in (with a password and TOTP, ADR 0017,
and the older ways), getting back into an account, the account's ways in and its
deletion, the pruning of what nobody can return to, what an instance tells its apps
before anyone signs in, and people's public profiles with their pages on the web, which
the instance's register keeps (ADR 0016). It comes from Halkyon Learn's server, whose
rules it keeps.

## What it provides

Endpoints, under the nickname `base` in a host's client (`client.modules.base.emailCode`):

| Endpoint | Methods | Who may call |
|---|---|---|
| `anonymousIdp` | `login` (Serverpod's anonymous sign-in, at most 30 new accounts per network and hour) | anyone |
| `password` | `set`: the caller's password, with the address a mailed code proves unless they have one; `beginChange`, `change`: a new password with the old one's proof and a code, every other session ending; `setRecoveryKeys`: a new recovery phrase's wrapping, with the old password's proof and a code | signed in |
| `passwordSignIn` | `begin`, `complete`: SRP-6a's two steps with a code from an authenticator (or a recovery code; a setup never finished, a code by mail); a session. `signUp`: an account at the address a mailed code proves, with a password; a session | without a session |
| `authenticator` | `enrol`, `confirm` (the first brings ten recovery codes; another takes a code from one confirmed before), `list`, `remove`, `newRecoveryCodes` | signed in |
| `accountRecovery` | `resetAuthenticator`, `beginKeyProof`, `resetPassword` (with the recovery phrase), `startAccountReset`, `completeAccountReset` | without a session |
| `gitHubUpgrade` | `upgrade`: links a GitHub account to the caller, who becomes `identified`; refused where passwords are offered (deprecated) | signed in |
| `gitHubSignIn` | `signIn`: a session for whoever has that GitHub account, unless they sign in with a password; none once `GITHUB_SIGN_IN_ENDS` has come | without a session |
| `emailCode` | `sendCode`: a code by mail, which proves the address and signs nobody in | anyone |
| `account` | `delete`, `signInWays` | signed in |
| `instance` | `basics`: the app it serves, GitHub's client ID if any and when GitHub sign-in ends, whether codes by mail and password sign-in are on, the source link, the oldest app build, where data and backups are | anyone |
| `profile` | `mine`, `save`, `checkHandle`, `checkLinks` (at most 10 an hour), `setAvatar`, `removeAvatar`, `remove`: the caller's own public profile | `identified` |
| `profiles` | `byHandle`: anyone's public profile, whatever the handle's case | signed in |

- GitHub keeps only the numeric GitHub ID, in Serverpod's own table; email keeps only
  HMAC-SHA256 of the address, keyed with `emailSignInPepper` (`email_identity`), and its
  codes as Argon2id hashes (`email_code_request`). Codes are eight Crockford characters,
  valid 15 minutes, 3 tries; up to 3 valid at once for an address (a fourth voids the
  oldest, so codes others ask for don't void the one its owner is typing); at most 3 an
  hour per address and network, 10 per address from every network (no mailbox flooded)
  and 10 per network.
- Profiles: a handle as Mastodon's (1 to 30 letters, digits or underscores, unique
  whatever their case, a few reserved for the instance), fixed once the host says so
  (`fixHandle`; Halkyon Learn at the first publish); a display name, a bio, up to 4
  links verified as Mastodon verifies them (`rel="me"` back to the profile's page,
  fetched over HTTPS from public addresses only), and an avatar, a PNG of at most 1 MB
  in the public storage at `avatars/<SHA-256>.png`. The page is at
  `https://<account domain>/@handle` (`ACCOUNT_DOMAIN`, else the server's public host),
  served by `addProfilePageRoute`, in English or German, its foot what the host's
  `profilePageNote` says.
- **Per-network limits** (new anonymous accounts, codes by mail, password sign-ins,
  proofs of the recovery phrase) count by the visitor's IP address (`networkOf`): the
  connection's, or, for a connection from a proxy the operator trusts
  (`TRUSTED_PROXIES`), the last address of `X-Forwarded-For`, the one the proxy saw
  itself. Never the `Forwarded` header, which Caddy passes on as the visitor wrote it,
  so never Serverpod's `remoteInfo`. An IPv6 visitor counts by their /64
  (`networkKeyOf`), which one line or server holds whole. Serverpod's own limit on
  anonymous accounts is off
  (it counts by the connection's address, behind a proxy the proxy's for everyone); the
  endpoint counts them itself.
- Refusals are `PasswordRefusedException`, `GitHubSignInRefusedException`,
  `EmailSignInRefusedException` and `ProfileRefusedException`, each with a reason;
  `ProfileNotFoundException` where there is no profile.
- For the host's own code, `package:halkyon_base_server/base.dart`: the configuration,
  `tokenManagerBuilders` and `identityProviderBuilders`, the `identified` scope and
  `grantIdentified`, `deletePerson`, the pruning (`pruneAbandonedAnonymousPeople`,
  `forgetOldSignUpAttempts`, `forgetExpiredEmailCodes`, `forgetUnconfirmedAuthenticators`,
  `forgetGitHubAccounts`, `forgetLapsedResets`), short codes, `sourceCodeUrl`,
  `dataLocation`, `backupLocation`, people's profiles as the host's code reads them
  (`profileOf`, `profilesOf`, `personWithHandle`, `fixHandle`; `saveProfile` for its
  tests), `accountDomain`, `profilePageUrl`, `addProfilePageRoute`, the register
  (`AccountRegister`, `AccountsMode`, `addInstanceRoutes`, `deliverDeletionNotices`,
  `deliverSessionNotices`, `forgetPeopleTheRegisterForgot`, `issueSession`), and the
  models. A host never reads
  the profiles' table itself.

## The instance's register (ADR 0016)

One operator's services share one account per person: the same address, GitHub account
and handle reach the same person in each, under one account ID (each service's
`AuthUser` has it), and deleting the account deletes it everywhere. One service holds
the register, its database keeping the ways in (`email_identity`, GitHub's IDs), the
handles and profiles; the others are its members and ask it. Anonymous people stay each
service's own until they add a way in, which makes them an account of the register under
the same ID. Sessions stay each service's own: every app signs in itself.

- **Modes** (`AccountsMode`, `HalkyonBase.accounts`; a host reads it with
  `AccountsMode.fromEnvironment()`): `HoldsRegister` by default, also for an instance of
  one service, telling the members `INSTANCE_MEMBERS` names (web servers, separated by
  commas or spaces) when an account goes; `MemberOfRegister` where
  `ACCOUNT_REGISTER_URL` names the register's web server. Empty values count as unset.
- **Trust:** the instance's shared secret (`instanceSecret` in the passwords), sent as a
  bearer token with every call between the services and compared in constant time. A
  server without one takes no such call and makes none (a member without one fails to
  ask anything). A member is trusted as the register's own code: whoever holds the secret
  can act for any account, so it never leaves the operator's servers.
- **The interface:** `AccountRegister`, which every endpoint and host function asks:
  `LocalRegister` on the register (its own tables), `RemoteRegister` on a member (a POST
  of JSON to `/halkyon-base/register/<operation>` on the register's web server, 30 s;
  refusals come back as the exceptions the register threw, anything else as
  `RegisterUnavailableException`); tests put an in-memory one in its place
  (`AccountRegister.use`). The register answers on its web server (`addInstanceRoutes`),
  a member on `/halkyon-base/member/account-gone`; neither is reachable from outside (the
  host's Caddyfile never forwards them).
- **Signing in on a member:** the app calls the same endpoints in both modes. The member
  asks the register to mail a code (the device's network passed on, as the register
  counts codes per network) and to check it, or to say whose GitHub ID it is, and opens a
  session of its own for that account (`issueSession`, which makes them a person there
  under the account's ID the first time). Linking a way in makes the person identified on
  the register and then on the member; a switch token signs in as the account the
  address is already. Every refusal is the register's, as on the register itself.
- **Deleting:** an identified person's deletion on a member goes to the register first
  (refused while it can't be reached, deleting nothing), which deletes the account and
  keeps a notice for each member in the same transaction (`account_deletion_notice`),
  told at once and again hourly (`deliverDeletionNotices`, tried least first) until each
  confirms; notices to a member no longer named are dropped. An anonymous person goes
  from their service alone. Each member also deletes, daily, the identified people the
  register no longer knows (`forgetPeopleTheRegisterForgot`): it asks about everyone
  before deleting anyone, deletes nobody while the register knows none of them (a
  register misnamed or emptied), and fails, deleting nobody, while it can't reach it.
  After restoring the register's database from a backup, accounts made since are
  unknown to it: stop the members' pruning until those people are dealt with.
- **Privacy:** the register learns when someone signs in to which service; the
  instance's privacy page has to say so.
- **The register being down:** members can't sign anyone in, change profiles or delete
  accounts; their open sessions go on working.

## SRP-6a (ADR 0017)

The password never reaches the register: the device proves it knows it with SRP-6a
(RFC 5054), and the register keeps only the verifier v = g^x mod N, which `halkyon_keys`'
x makes on the device (Argon2id, so a stolen verifier costs an Argon2id per guess). The
two halves share the arithmetic, generic over the group and the hash (`SrpGroup`):

- the register's in `package:halkyon_base_server/base.dart`: `srpChallenge(group,
  verifier)` gives `SrpChallenge` (B for the client; b kept for this sign-in only), and
  `srpVerify(group, identity:, salt:, verifier:, challenge:, clientValue:, clientProof:)`
  gives `SrpAccepted` (M2 and K) or `SrpRefused` (`invalidClientValue`, `wrongProof`);
- the device's in `package:halkyon_base_client/srp.dart`, written by hand beside the
  generated client (the generator writes only `lib/src/protocol/`): `SrpClient(group)`
  with its `publicValue` A, `respond(identity:, salt:, privateValue:, serverValue:)`
  giving `SrpClientProof` (M1, K, `verifiesServer(M2)`) or `SrpClientRefused`, and
  `srpVerifier(group, x)` for sign-up.

Production uses `SrpGroup.rfc5054Group3072` (RFC 5054's 3072-bit group, g = 5) with
SHA-256. k = H(N | PAD(g)), u = H(PAD(A) | PAD(B)), K = H(PAD(S)),
M1 = H(H(N) xor H(g) | H(I) | s | PAD(A) | PAD(B) | K), M2 = H(PAD(A) | M1 | K), numbers as
unsigned big-endian bytes, PAD to N's length. A and B must lie in 1..N−1 (so neither is
≡ 0 mod N) and u must not be 0; a and b are 256 bits from `Random.secure()`; proofs are
compared in constant time. RFC 5054 Appendix B's vectors check both halves
(`SrpGroup.rfc5054TestVectorGroup`, SHA-1), and `test/unit/srp/srp_interop_test.dart` runs
the client against the register, every refusal included.

## TOTP and recovery codes (ADR 0017)

The second factor, in `package:halkyon_base_server/base.dart`:

- `newTotpSecret()`: 160 random bits, shown to the person in RFC 4648 Base32
  (`base32Encode`, `base32Decode`) through `otpauthUri(issuer:, account:, secret:)`, its
  label and issuer escaped (a space as `%20`).
- `checkTotp(secret:, code:, now:, lastAcceptedStep:)`: RFC 6238 with HMAC-SHA1, 6 digits,
  30-second steps, one step either side of now. The account's last accepted step is a
  required argument, and a code from that step or an earlier one is refused (`reused`),
  so no code is accepted twice; `TotpAccepted.step` is the step to keep before answering.
  Refusals: `malformed`, `wrong`, `reused`. `totpCode` makes a code (SHA-256 and SHA-512
  for RFC 6238's vectors).
- `RecoveryCodes.generate(key)`: ten codes of eight Crockford characters (`K7QX-M2PD`, as
  email sign-in's codes), shown once, kept as HMAC-SHA256 under the instance's key
  (`recoveryCodeKey`, at least 32 characters, kept outside the database: a code's 40 bits
  in a leaked database give nothing to guess against without it); `useRecoveryCode(typed,
  hashes, key)` compares every hash in constant time and gives the hashes left
  (`RecoveryCodeAccepted.remaining`), so each code works once.

## Signing in with a password (ADR 0017)

Built on the three parts above, on the register (members forward every step through
`AccountRegister`, `RemoteRegister` included; the session is the member's own).

- **Tables** (`serverOnly`, each row deleted with its person by `deletePerson`):
  `account_password` (one per account: Argon2id's salt and costs, SRP's verifier padded
  to N, the public keys, the private keys wrapped under the password and under the
  recovery phrase), `totp_authenticator` (the secret sealed with XChaCha20-Poly1305 under
  a key derived from `totpKey`, bound to the row's account and ID; `confirmedAt`;
  `lastStep`), `recovery_code` (keyed hashes), `sign_in_challenge` (a step begun: SRP's b
  and B, or a nonce to sign; two minutes, taken once by a delete), `account_reset` (one
  of each kind per account, with when it takes effect).
- **Setting a password** (`password.set`): the device sends salt, costs (`argon2id13`,
  3–16 passes, 64 MiB–1 GiB), verifier (1 < v < N, as long as N), public keys (32 bytes
  each) and both wrappings (24-byte nonce, 112 bytes). It needs an address, and a
  session alone sets none (ADR 0017's fourth reading): an account with an address proves
  it anew by a code mailed to it for this (`firstPassword`); an anonymous person by a
  mailed code (`signUp`) for the address they add, which becomes theirs as email sign-in
  makes it (anonymous → `identified`, same ID); an account that signs in with GitHub
  alone does that within 10 minutes of a GitHub sign-in on the session asking
  (`gitHubSignInFreshFor`, the session row's method and creation time, a member telling
  the register; else `signInAgain`, before any code is used). It signs in once an
  authenticator is confirmed; until then a new setup replaces it, never once one was,
  even while a reset leaves the account without one (a new password comes through the
  recovery phrase or the account's reset alone).
- **Codes by mail** (`emailCode.sendCode`, without a session) only prove an address: for
  a password's setup, a sign-up, a setup never finished, the resets and deleting an
  account without a working second factor. Each is asked for a purpose
  (`EmailCodePurpose`: `signUp`, `finishSetup`, `authenticatorReset`, `accountReset`,
  `deletion`; `email_code_request.purpose`), only that purpose takes it (any other is
  refused as no longer valid and leaves it unused), and its mail says what it would do,
  in its subject too (a notification shows the subject alone), and to type it only in
  the app on that instance. Since step 3
  nothing turns a code into a session (`emailCode.verifyCode`, `emailSignIn.signIn` and
  the switch token went with build 22, `minimumAppBuild` 22 in Learn).
- **Signing up** (`passwordSignIn.signUp(request, code, setup)`, without a session): an
  address nobody has becomes a new identified account, an account with the address and
  no password takes it (keeping its ID); refused (`passwordExists`) for an address whose
  account has a password. The session it gives sets up the authenticator.
- **Signing in:** `begin(address)` answers salt, costs and B; `complete(challenge, A, M1,
  totpCode | recoveryCode)` answers M2, the public keys, the password's wrapping and a
  session. A setup never finished (never a confirmed authenticator) signs in with a code
  mailed to the address it began with in its place (`request`, `code`), which calls off
  a reset of the account waiting; any other account takes no code by mail there and
  leaves it unused. SRP's identity I is `passwordIdentity`: the challenge's ID, the
  instance's base URL the device addressed (`instanceUrl` on every proving call, written
  alike on both sides by `canonicalInstanceUrl`: in production it must be the server's
  own public scheme, host and port at the root, `publicInstanceUrlOf`, else
  `otherInstance`, so an instance served under a path takes no password sign-in; in
  development any, as an emulator reaches the machine as 10.0.2.2), what the proof is for
  and the digest of what it sets (`PasswordProofPurpose`; signing in and an
  authenticator's reset each its own, with none), so a hostile instance relaying a
  sign-in or a phrase's proof here gets nothing taken, nor one beside it on the same host;
  a member forwards its own base URL, and the phrase's proof for a new password signs
  that URL and that password's digest (`newPasswordDigest`). An address nobody has (or whose
  account has no password) gets a salt and verifier from `signInDecoyKey` and the
  address's hash, the same queries and arithmetic, and the same refusal: a wrong
  password, an unknown address and a wrong, used or missing code are all
  `wrongCredentials`; `setupIncomplete` and `notDue` come only after a right password.
  At most 10 sign-ins begun per address and network an hour (5 proofs of the phrase),
  30 per network, and, from every network together, 30 proofs checked per address
  (`proofsPerAddress`: every password and phrase proof, recorded before it is checked
  whatever its outcome, so a right one counts as a wrong one and challenges begun ahead
  get no more checked; an address nobody has alike), past which no proof is checked and
  nothing begins for it anywhere until the hour has passed: so whoever knows an address
  and fills its count from one network holds no other back, and guessing from many
  makes at most 30 guesses an hour. A signed-in owner's proofs (a `change` challenge,
  below) never count toward the address, and no sign-in takes one. A sign-in or the
  phrase's reset carrying a code takes the account's `address` and `language`, refused
  (`wrongCredentials`) before any code is checked when it isn't theirs.
- **The owner's proof** (`OwnerProof`, ADR 0017's third reading, as Proton has it): a
  session alone never gets a code checked. Every signed-in change of how an account is
  secured (adding an authenticator once one is confirmed, removing a confirmed one, new
  recovery codes, a new phrase, a new password, deleting) takes the password's SRP proof
  for a challenge `password.beginChange` gave (30 begun an hour per session, holding
  back no other) and then a TOTP or recovery code (`proveOwner`, `ownersCode`). A proof
  given where none is taken is refused, so none passes unchecked. The proof is bound to
  what it approves: SRP's identity carries the purpose and the digest of what is set
  (`newPasswordDigest` of the new password, `newPhraseDigest` of the phrase's wrapping
  and proof key, `authenticatorDigest` of the authenticator confirmed or removed; none
  for new recovery codes, calling resets off and deleting), which the register works out
  from the call itself, so whoever stands between swaps nothing in and a proof made for
  one change makes no other. Each proof counts for
  its session alone (`owner_proof_tries.dart`, `asOwnerProofTry`, table
  `session_code_failures`, kept where the session is, on a member too), before it is
  checked; the tenth wrong in a row ends the session (`sessionEnded`), only a proof
  accepted clears the count, and a try that checked none is taken back. Never toward the
  account's waits, nor waiting for them: whoever knows the password holds no signed-in
  owner out of changing it, and a new password forgets those waits. A session ended so
  warns its owner: the register keeps the moment (`account_password`'s
  `sessionEndedForProofsAt`; a member's ended session tells it through
  `noteSessionEndedForProofs`, a register out of reach leaving the owner unwarned), which
  `account.signInWays` and the next `passwordSignIn.complete` carry until the owner
  dismisses it (`password.dismissSessionEndedWarning`, the session alone: it hides a
  warning) or sets a new password.
- **Authenticators:** a secret made on the register, shown once as an `otpauth://` URI
  (issuer: the account domain); confirmed by a code from it and the owner's proof, so a
  session alone adds no second factor: while the account has a confirmed one, the
  password and a code from it; the first (none confirmed, after a reset carried out
  too) the password alone (`proveOwnersPassword`, the fourth reading); confirming one confirmed already is refused as unknown (its code is
  never checked for a session); the first confirmed brings ten recovery codes; at most
  10 per account; the last confirmed one stays; removing a confirmed one, or new recovery
  codes, takes the owner's proof, removing one never confirmed nothing. Each step's code
  is accepted once, by a conditional update of `lastStep`.
- **Codes tried in a row** (`second_factor_waits.dart`, table
  `second_factor_failures`): every code tried against an account's second factor after
  a right password or phrase at sign-in or in the phrase's reset (never the owner's
  proofs while signed in) counts, per account and way it came (`SecondFactorPath`:
  after the password, after the phrase, each with its own waits), from every network,
  before it is checked, one at a time per account (an advisory lock), so codes sent at once get no more checked; every tenth
  in a row begins a wait as it is counted (`codesWait`, with when), an hour, then twice
  as long each time, a day at the most, and every wait mails the account's address
  (which those calls take); a code accepted, either reset carried out, and a new
  password clear it.
- **Resets:** a lost authenticator: SRP's proof plus a mailed code or the recovery
  phrase's signature asks for it; it takes effect 72 hours later, a mail tells the
  address at once, and signing in with a code calls it off; the sign-in that carries it
  out ends every session of the account in every service (its own session comes after
  that moment), and then the password alone signs in until a new authenticator is
  confirmed. A lost password with the phrase: a
  signature over a nonce (`keyProofMessage`) by the phrase's own proof key (Ed25519,
  derived from the phrase alone, its public half kept as `recoveryProofKey`; never the
  account's signing key, which every signed-in device keeps) and a code set a new
  password, the keys staying. A lost password without it: a mailed code asks, 72 hours
  pass (a mail at the start, a sign-in calls it off), a new code carries it out: new
  password, new keys, no authenticators. A reset takes effect once the owner had 72 hours
  outside hold-outs (codes waiting, the address's ceiling of proofs filled): each
  hold-out that begins before it takes effect moves it on by the time it holds the owner
  out and no hold-out counted before did (`pushOutResets`, `account_resets.dart`, the
  row's `heldOutUntil`); a reset that took effect and wasn't carried out lapses a week
  later (`resetLapsesAfter`), and every reset not carried out 30 days after it was asked
  (`resetLivesAtMost`), both pruned by `forgetLapsedResets`; `SignInWays` tells
  signed-in devices when each waiting reset takes effect, and `password.callOffResets`
  calls them off for the owner's proof.
- **Changing while signed in:** `password.change` takes the owner's proof (the old
  password's SRP proof for a `password.beginChange` challenge, and a TOTP or recovery
  code) and the same keys wrapped under the new password; `password.setRecoveryKeys`
  replaces the phrase's wrapping with the same proof (a phrase of one's own would
  otherwise set a new password through the phrase's reset). A session alone changes
  neither, nor a session and a code.
- **Deleting an account with a password** (`account.delete(proof:)`, a `DeletionProof`)
  takes the same: the password's proof for a `beginChange` challenge and a code, bound
  to the base URL the device addressed and made for deleting (`requireOwnerToDelete`; a member passes the proof on
  and the register checks it), counted for the session as every owner's proof is. The
  code is a TOTP or recovery code wherever an authenticator is confirmed; where no second
  factor works (a setup never finished, or the authenticators' reset taken effect;
  `SignInWays.secondFactorWorks` tells the app) it may be one by mail asked for deleting
  (`request`, `code`), to the account's own address. An account without a password
  deletes with its session, as before.
- **Sessions end everywhere:** a change of password ends every other session of the
  account (a setup's password replaced too); a reset of the password or the account ends
  all. The register ends its own and keeps a `sessions_ended_notice` per member (told at
  once in the background, and hourly until it confirms, `deliverSessionNotices`, as deletions are told): it names a moment, so a
  session made since stays, and the session that asked for a change. A member answering
  one (`/halkyon-base/member/sessions-ended`) ends its sessions made before the moment,
  taken by its own clock (the notice carries the register's, `sentAt`, so the clocks'
  offset drops out); a member signing someone in after a reset ends every session of the
  account it holds made before the register's moment or before now, whichever is later,
  then makes the new one (`issueSessionAfter`), waiting for nothing. The owner's warning of a session ended for its proofs is
  dismissed only by a session made since (`signedInBefore`). A refusal for a reset not due says when it takes effect
  (`takesEffectAt`).
- **Older ways:** an account that signs in with a password (it had a confirmed
  authenticator, which a reset may have taken since: `passwordOnly` in its ways in) no
  longer signs in with GitHub (`usePassword`); where passwords are offered, no new GitHub
  account is linked (`deprecated`); from `GITHUB_SIGN_IN_ENDS` on GitHub signs nobody in
  (`ended`), the basics stop naming the day (`gitHubSignInEnds`, which apps show until
  then) and, 30 days later (`gitHubAccountsKeptAfterEnd`, so a date set wrong can be put
  right first), `forgetGitHubAccounts` deletes every GitHub ID on the register, first
  deleting the accounts that sign in with GitHub alone (no address, never a password),
  each on its own (one failing is tried again the next day), the members told once
  after all of them. `password.set` adds an
  address without GitHub only for someone anonymous where the session lives
  (`anonymousHere`, which a member tells the register).

## Taking it into a host

1. Depend on both packages by path, within the repository's workspace (an app's
   packages sit in `apps/<app>/`):

   ```yaml
   # the server's pubspec.yaml
   halkyon_base_server:
     path: ../../../packages/halkyon_base_server
   # the client's pubspec.yaml: the same with halkyon_base_client
   ```

2. Name it in the server's `config/generator.yaml`:

   ```yaml
   modules:
     halkyon_base:
       nickname: base
   ```

3. Configure it once, before the pod starts, and run its authentication and pruning:

   ```dart
   final pod = Serverpod(args); // the host's generated Serverpod
   HalkyonBase.configure(
     pod,
     HalkyonBase(
       app: 'todo', // its apps accept only instances naming it; never change it
       appName: 'Halkyon Todo', // in sign-in mails
       upstreamSourceCodeUrl: 'https://code.halkyon.app/project-halkyon/halkyon_todo',
       minimumAppBuild: 1, // raise it in the commit that breaks older apps
       deletePersonData: [deleteTodosOf], // the host's own data of a person
       profilePageNote: ( // what profile pages say of the app at their foot
         english: 'On Halkyon Todo, a to-do list:',
         german: 'Auf Halkyon Todo, einer Aufgabenliste:',
         link: Uri.parse('https://halkyon.app/todo'),
       ),
       // The register, or a member of the one the operator names (ADR 0016).
       accounts: AccountsMode.fromEnvironment(),
     ),
   );
   pod.initializeAuthServices(
     tokenManagerBuilders: tokenManagerBuilders,
     identityProviderBuilders: identityProviderBuilders(pod),
   );
   ```

   Serve the calls between the instance's services on its web server:
   `addInstanceRoutes(pod)`; and the profile pages: `addProfilePageRoute(pod)`, after any
   literal routes of its own.

   Then schedule the pruning in a future call of the host's own: daily
   `pruneAbandonedAnonymousPeople`, `forgetGitHubAccounts` (the register's),
   `forgetLapsedResets` and `forgetPeopleTheRegisterForgot` (a member's), hourly
   `forgetOldSignUpAttempts`, `forgetExpiredEmailCodes`,
   `forgetUnconfirmedAuthenticators`, `deliverDeletionNotices` and
   `deliverSessionNotices` (the register's). Each marked as one mode's does nothing in
   the other.

4. A host's deletion (`PersonDataDeletion`) runs inside `deletePerson`'s transaction,
   after the person's sessions ended and before their identities and the person go, in
   the order given; it deletes the person's rows in the host's tables, and returns what
   has to wait until the deletion is committed (files outside the database), or null.
   Pruning deletes through the same function.

5. Create the host's migration. A host taking the tables over from its own models (as
   Halkyon Learn does) finds no change once its copies are gone: Serverpod matches tables
   by name, and these are identical. `create-migration --empty` then records the module's
   version, with no SQL besides. Every service has the module's tables, whichever its
   mode: a member's stay empty but for its people's sessions.

The passwords it reads: `serverSideSessionKeyHashPepper`; for GitHub `githubClientId` and
`githubClientSecret`; for email `emailSignInPepper` and `emailSmtpHost`,
`emailSmtpPort`, `emailSmtpUser`, `emailSmtpPassword`, `emailFrom`; for passwords
`totpKey`, `recoveryCodeKey` and `signInDecoyKey` (each at least 32 characters, or
`addInstanceRoutes` stops the server; all three and email sign-in, or no password
sign-in); between the instance's services `instanceSecret`. Never change either pepper
once people sign in: every session or address hash would be lost; changing `totpKey`
voids every authenticator, `recoveryCodeKey` every recovery code. A member needs no email settings of its
own (the register mails the codes), but GitHub's (its own OAuth app's, whose callback is
its app's). The environment: `ACCOUNT_REGISTER_URL` and `INSTANCE_MEMBERS` (above);
`ACCOUNT_DOMAIN`, the domain handles are on, which can never change once anyone follows
one; `GITHUB_SIGN_IN_ENDS`, the day GitHub stops signing anyone in (`2027-01-22`);
`SOURCE_URL`, `DATA_LOCATION`, `BACKUP_LOCATION`: where the instance keeps people's data
and its backups, each `EU`, a country's code (`DE`) or a subdivision's (ISO 3166-2,
`US-CA`, `DE-BY`), the backups also `none`; where the disks are, not which law governs;
`TRUSTED_PROXIES`, the addresses of the reverse proxies in front of the server (Caddy's
on its compose network, which Halkyon Learn's `deploy/compose.yaml` sets), separated by
commas or spaces: without, every visitor behind the proxy shares each per-network limit;
anything but addresses stops the server (`addInstanceRoutes`).

## Developing it

`serverpod --no-analytics generate` here after changing a model or an endpoint, and
`serverpod --no-analytics create-migration` after changing a table. Tests need no Docker:
`dart test` here starts Serverpod's embedded PostgreSQL. They read `config/passwords.yaml`
(git-ignored) with throwaway values under `test:` for `database`, `serviceSecret`,
`serverSideSessionKeyHashPepper` and `emailSignInPepper`. Password sign-in's keys may be
there too (CI's are), but the tests set their own (`usePasswordKeys`) and switch them
off otherwise.

## Licences

The server package is under the Server Side Public License 1.0, as the Halkyon servers
are; the client package under the GNU Affero General Public License 3.0 or later, as the
Halkyon apps are. So each combines with its counterpart in a host exactly as the host's
own do.