feat(accounts): one account across an instance's services (ADR 0016)

halkyon_base runs as the instance's register or as one of its members.
The register keeps the ways in, handles and profiles; members ask it
over HTTP with the instance's shared secret (instanceSecret), and open
sessions of their own for its accounts, under the same account ID.

- AccountsMode from the environment: INSTANCE_MEMBERS on the register,
  ACCOUNT_REGISTER_URL on a member. Learn always holds the register
  (Explore joins the profiles); Todo is a member where it is set.
- AccountRegister: LocalRegister, RemoteRegister, an in-memory one in
  tests. Calls between services on the web server, refused without the
  secret (constant-time compare); a server needing it doesn't start
  without it.
- Deleting an account deletes it everywhere: notices per member kept in
  the deletion's transaction (account_deletion_notice), told at once and
  hourly until confirmed; members prune daily whom the register no longer
  knows, never while it knows none of them or can't be reached.
- Deploy: INSTANCE_SECRET and INSTANCE_MEMBERS in compose and .env.

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

apps/todo/AGENTS.md

@@ -3,7 +3,7 @@
 What holds for every app and package of this repository is in the [root AGENTS.md](../../AGENTS.md); it applies here in full, and Learn's patterns (`apps/learn/AGENTS.md`) wherever Todo needs the same thing.
 
 - **A shell on the shared packages** (decided 2026-10-19): signing in, the instances, the account and the look work; nothing of a person's is kept or synced yet, which waits for Todo's first model (the developer decides what it keeps and syncs). Packages: `halkyon_todo_server`, `halkyon_todo_client` (generated, never edited), `halkyon_todo_flutter`; app ID `app.halkyon.todo` on every platform; no web build. Licences as Learn's (server SSPL, client and app AGPL; `LICENSE.md`).
-- **The server** includes `halkyon_base` (ADR 0014) and configures it in `server.dart` (`todoBase()`, `lib/src/todo_base.dart`): its basics say it serves `todo` (never change it once apps use it), sign-in mails name Halkyon Todo, the source is this repository, `minimumAppBuild` 1, no data of its own to delete with a person yet (add a `deletePersonData` hook with Todo's first table). Authentication through the module's builders: anonymous, GitHub (with the instance's own OAuth app) and email codes, server-side sessions. `schedulePruning` (`lib/src/pruning.dart`) keeps the module's pruning scheduled, one entry each however often the server starts. Session logs are never persisted, in any run mode. One migration so far, the initial one (2026-10-19): Serverpod's tables, its auth modules' and the module's (`email_identity`, `email_code_request`), nothing of Todo's own. Tests (`test/integration/todo_server_test.dart`) call the module's endpoints through the server's own dispatch (`halkyon_base.<endpoint>`), since the generated test tools expose the project's endpoints alone.
+- **The server** includes `halkyon_base` (ADR 0014) and configures it in `server.dart` (`todoBase()`, `lib/src/todo_base.dart`): its basics say it serves `todo` (never change it once apps use it), sign-in mails name Halkyon Todo, the source is this repository, `minimumAppBuild` 1, no data of its own to delete with a person yet (add a `deletePersonData` hook with Todo's first table). Authentication through the module's builders: anonymous, GitHub (with the instance's own OAuth app) and email codes, server-side sessions. **Accounts** (ADR 0016): Todo holds its own register when it runs alone, and is a member of the instance's register (Learn's server on learn.halkyon.eu) where the operator sets `ACCOUNT_REGISTER_URL` (`todoBase(accounts: AccountsMode.fromEnvironment())`) with the instance's shared `INSTANCE_SECRET` (`instanceSecret`); then sign-in, handles and profiles are the register's, the email settings are the register's alone (GitHub's are Todo's own OAuth app's), and an identified person's deletion goes to the register first. `addInstanceRoutes` serves the calls between the services on its web server, and the server doesn't start without the secret where it needs it. `schedulePruning` (`lib/src/pruning.dart`) keeps the module's pruning scheduled, one entry each however often the server starts: anonymous people daily, sign-up attempts and codes hourly, as the register the deletions its members haven't confirmed hourly, as a member the people the register no longer knows daily (each a no-op in the other mode). Session logs are never persisted, in any run mode. Migrations (2026-10-19): the initial one (Serverpod's tables, its auth modules' and the module's `email_identity`, `email_code_request`), `profiles` (`public_profile`) and `deletion-notices` (`account_deletion_notice`), nothing of Todo's own. Tests (`test/integration/todo_server_test.dart`) call the module's endpoints through the server's own dispatch (`halkyon_base.<endpoint>`), since the generated test tools expose the project's endpoints alone.
 - **The app** is halkyon_app's `InstanceGate` and `SessionGate` with Todo's frame (`todoFrame`: "Todo" under "Halkyon", its icon, `todo.example.org`, its own three sentences) and `ServerpodInstanceChecker(app: 'todo')`; `TodoDependencies` (`AppDependencies<TodoPerson>`) on one instance, `TodoPerson` (empty) per person; a person's app with an empty home and the account sheet (whose account on which instance, Switch account, Sign out, which asks first for an anonymous account as it deletes it, Delete account, which asks first, and About with the source of what the instance runs and the licence list). Texts through gen-l10n in British English (the template), German (informal du), an empty `en_US` and `en_GB` (`test/l10n/arb_files_test.dart` keeps them in step). The look follows the system's theme mode and language until Todo has settings. Countries in where an instance keeps its data show by their codes until halkyon_app names countries itself. Widget tests start Todo's whole tree with halkyon_app's fakes (`test/app/todo_app_test.dart`).
 - **The icon** is Lucide's `list-checks` (lucide-static 1.50.0) in the brand's blues on white: `apps/todo/tool/app_icons.py` (run from `apps/todo`) fetches it, draws its strokes with Pillow (no SVG renderer needed; other path commands fail loudly), writes `assets/icon/halkyon_todo_icon.png` and Lucide's ISC licence beside it, then every platform's launcher icon through flutter_launcher_icons as Learn's tool does. `registerLucideLicense` puts the notice in the licence list.
 - **`serverpod start`** runs in `apps/todo/halkyon_todo_server` (it launches the app from `lib/driver.dart`); `.mcp.json`'s `--server-dir` names it when a session works on Todo. Its tests need throwaway passwords in its git-ignored `config/passwords.yaml` (`database`, `serviceSecret`, `serverSideSessionKeyHashPepper`, `emailSignInPepper`) or from the environment, as CI gives them. Learn's and Todo's development servers use the same ports: run one at a time.