feat(base,learn,todo): the operator's assistants, and the instance's privacy page (ADR 0021, step 2)

The server half of the assistants (points 4 to 8, D27 to D46): the
operator sets language models in deploy/assistants.env, grants them to
people with limits, and the module forwards their questions with the
operator's key, counting what each used, never what was asked.

- assistantsFrom: ASSISTANTS, ASSISTANT_ID_KEY, ASSISTANT_<ID>_NAME,
  _PROTOCOL (anthropic, openai, openai-responses), _BASE_URL, _MODEL,
  _KEY, _IMAGES, _PRICE_INPUT and _PRICE_OUTPUT (per million tokens),
  ASSISTANT_CURRENCY, ASSISTANT_OWN_KEYS; anything unusable stops the
  server as it starts, naming the variable.
- The assistant endpoint (identified): offered, agree (D40), ask as a
  Serverpod stream (D35): the text as the model writes it, then the
  whole turn, sent once the request is counted; figures, the caller's
  own. Checked in order: at most 2 questions on their way per person
  (tooManyAtOnce), the grant, the agreement, limits in the pricing's
  unit now (limitOutdated), the month's and the day's limits (with when
  they reset, UTC), the question (malformed, imagesNotRead, tooLarge
  past 400,000 characters, an image over 3,932,160 bytes or more than
  20 images); output capped at 16,384 tokens. Each request carries the
  person's opaque ID (HMAC-SHA256 under ASSISTANT_ID_KEY, D30). Once
  sent, the provider's reply is read to its end even if the app
  cancels, and counted on a session of its own; a provider's failure is
  logged by kind and status, told as providerKey, providerBusy (with
  its wait), providerRefused or providerFailed.
- The assistantOperator endpoint (identified and operator, on the
  register): setUp, grants, grant (assistantNotSet, invalidLimit,
  notFound), revoke, figures of a UTC month.
- Tables assistant_grant and assistant_figures (per person, assistant
  and UTC day), in the module's, Learn's and Todo's migrations. The
  daily pruning keeps this month and the two before
  (forgetOldAssistantFigures, D41); deletePerson deletes both.
  A member offers no assistant (D43).
- instance.basics() gains assistantOwnKeys.
- /privacy (D39, addPrivacyPageRoute, on the register's web server,
  before the profile pages' route): made from the instance's settings,
  English or German, no JavaScript: who may sign up, where the data and
  backups are, what the module keeps (the account, sessions, IP
  addresses, the server's log), the host's privacyPagePoints (Learn's:
  studysets synced, private ones encrypted on the devices with what
  stays readable, what is published, dictionary lookups, the learner
  count), each assistant with its model and host, what goes and what is
  counted, and whether personal keys are allowed. Never a key.

Tests: 70 on the assistants (grants, limits, refusals in order,
streaming, cancelling, counting, concurrency, the operator's denials,
deletion, pruning), 19 on the settings, 8 on the privacy page.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

AGENTS.md

@@ -9,7 +9,7 @@ Halkyon's apps and the packages they share, in one repository and one pub worksp
 - **`apps/<app>/`**: each app's packages, as `serverpod create` lays them out (`halkyon_<app>_server`, `_client`, `_flutter`), plus what is the app's alone (Learn's curator, the scripts writing Learn's assets in `apps/learn/tool/`, run from `apps/learn`). Learn is the app that runs; Todo is still Serverpod's scaffold; Train and Meditate are to come.
 - **`packages/`**: what the apps share, each with its README and licence, which the apps take by path (`path: ../../../packages/<package>`), so a change to a shared package and to the apps using it is one commit:
   - `halkyon_ui`: Halkyon's look, the theme around cobalt and copper, Noto Sans bundled, the window-size classes and the widgets every screen is built of, with texts of its own (English, German).
-  - `halkyon_base_server`, `halkyon_base_client`: the Serverpod module every Halkyon server includes (ADR 0014): signing in (a password with TOTP and the resets of ADR 0017, SimpleLogin on `halkyon_oidc`, email codes proving an address; no anonymous accounts since ADR 0020, D25, nor GitHub since D36), an account's ways in and its deletion with a hook for each app's own data, the pruning, the instance's basics, people's handles and public profiles, who may sign up (`SIGN_UP`: open, approval, closed) with the accounts waiting for approval and the instance's operators who approve them (Halkyon Learn's ADR 0021, step 1), and the instance's register (ADR 0016): one account per person across an operator's services, held by one service (Learn's server on learn.halkyon.eu) and asked by the others, its members (`ACCOUNT_REGISTER_URL`), all sharing a secret (`INSTANCE_SECRET`); its README says how.
+  - `halkyon_base_server`, `halkyon_base_client`: the Serverpod module every Halkyon server includes (ADR 0014): signing in (a password with TOTP and the resets of ADR 0017, SimpleLogin on `halkyon_oidc`, email codes proving an address; no anonymous accounts since ADR 0020, D25, nor GitHub since D36), an account's ways in and its deletion with a hook for each app's own data, the pruning, the instance's basics, people's handles and public profiles, who may sign up (`SIGN_UP`: open, approval, closed) with the accounts waiting for approval and the instance's operators who approve them (Halkyon Learn's ADR 0021, step 1), the operator's assistants forwarded on halkyon_llm with their grants, limits and counted figures, and the instance's privacy page made from its settings (step 2), and the instance's register (ADR 0016): one account per person across an operator's services, held by one service (Learn's server on learn.halkyon.eu) and asked by the others, its members (`ACCOUNT_REGISTER_URL`), all sharing a secret (`INSTANCE_SECRET`); its README says how.
   - `halkyon_app`: every app's shell on the module's client (ADR 0014): the instances a device keeps accounts on and the first start, signing in and switching person, the gates building an app's dependencies per instance and per person, the sync service and where it stands, the account section of an app's settings (`AccountTiles`), the wait screen of an account waiting for approval and the operator's card, badge and screen (ADR 0021), About with the instance's source link (`showAppAbout`), the language every app picks (`resolveAppLocale`) and countries' and subdivisions' names (Unicode CLDR's, bundled), with texts that name the app.
   - `halkyon_core` (low-level primitives in pure Dart: ids, a logger, `SerialQueue`, `CoalescingRunner`) and `halkyon_interop` (contracts between apps, pure Dart).
   - `halkyon_keys` (pure Dart on libsodium, ADR 0017): an account's keys on the device, the password's Argon2id and SRP's x, the key pairs and their wrapping, the recovery phrase, data keys, content sealed and signed. SRP-6a's two halves are the module's (`halkyon_base_server`'s `base.dart`, `halkyon_base_client`'s `srp.dart`), as are TOTP and the recovery codes. `sodium`'s build hook compiles libsodium, so every machine that builds or tests needs a C compiler and `make`.