feat(base,learn): spaces people share, kept by halkyon_base

ADR 0044, step 2 (D1, D2, D11, D13, and D15 to D17, decided today).
halkyon_base keeps the spaces several people share, for every app:
shared_space, its members with their roles (Owner, Editor, Viewer), and
invitations by handle waiting for someone with no account on the
service yet. The sharedSpace endpoint creates a space, lists its
members, invites by handle, changes roles, removes members, hands the
space on, leaves and deletes it.

Who may do what:
- Only people with an identity reach the endpoint.
- A space the caller isn't in can't be told from one that doesn't
  exist.
- Members are named by their membership, never by their account's ID.
- Only the Owner manages the members, unless the space lets members
  invite.
- The Owner's role passes only by handing the space on, and the Owner
  leaves only naming who takes it on.
- An invitation counts as an answer about a handle, as it tells whether
  one exists.
Every rule has a test asserting the refusal.

The module stays free of the sync engine: a host hands it a SpaceGrants,
and every call runs in the module's transaction, so the engine's grants
follow the roles or nothing changes. The Owner and an Editor write; a
Viewer reads. Learn hands over SyncSpaceGrants on OfflineSyncSpaces. Todo
hands over none yet, so its server refuses until its projects are
shared.

Deleting an account hands each space the person owns to the Editor who
joined first, else the Viewer who joined first. A space with no other
member goes. This runs before the host's own deletions. A person's first
sign-in on a service makes their waiting invitations memberships, in the
same transaction that makes them a person there.

Migrations: the module's and every host's (Learn, Todo, Train, Meditate)
create the three tables, their indexes and the two cascading foreign
keys to shared_space, and nothing else. The old server keeps working.

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`; the apps' icons are the root's `tool/app_icons.py`, ADR 0031). Learn is the app that runs; Train, Todo and Meditate run too (ADR 0030, ADR 0033, ADR 0038), Todo and Meditate not released yet.
 - **`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 (the navigation bar flush with the foot with its fade, and the rail, a banner above every tab, `NavigationFrame.banner`, `FoldingTabs`' tab row folding into the title), 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), the operator's assistants forwarded on halkyon_llm with their grants, each person's budget, the efforts they allow and counted figures, and the instance's privacy page made from its settings (step 2), people's files, kept by space and SHA-256 in the server's private storage with the room each may take and the pruning, for a host that keeps them (`HalkyonBase.files`, `FileStore`: which of its rows name a file and what it takes; Halkyon's ADR 0043; Learn's `asset` rows, Todo's attachments), 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, each person's budget, the efforts they allow and counted figures, and the instance's privacy page made from its settings (step 2), people's files, kept by space and SHA-256 in the server's private storage with the room each may take and the pruning, for a host that keeps them (`HalkyonBase.files`, `FileStore`: which of its rows name a file and what it takes; Halkyon's ADR 0043; Learn's `asset` rows, Todo's attachments), the spaces people share, with their members, roles (Owner, Editor, Viewer) and invitations, kept in step with the host's sync engine through its `SpaceGrants` (`HalkyonBase.spaces`, Halkyon's ADR 0044, step 2: Learn's `SyncSpaceGrants`; handed on as an Owner's account goes), 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, held back on a metered network as the device's `MobileDataGate` allows (ADR 0042: Android's flag through halkyon_app's own Android plugin, `HalkyonAppPlugin.java`, in Java so that it applies no Kotlin Gradle Plugin, `FakeNetworkWatch` in `testing.dart`; `mobileDataBannerOf` above every tab in each app's frame; the setting in `SyncStatusCard`; an app's files ask `SyncService.filesAllowed`), the account section of an app's settings (`AccountTiles`), every app's Profile (`HalkyonProfile`, decided 2026-11-02: the person's own tab, the same in every app, with their account, the ways they sign in and their public profile, one for all apps, then the app's own tabs) and people's public profiles (`PublicProfileRepository`, `PublicProfileSection`, `showProfileOf`), the wait screen of an account waiting for approval and the operator's card, badge and screen (ADR 0021), the assistants of an app's settings (`AssistantsCard`: granted, by provider, agreed to, what is left of the person's budget, used, and the person's own keys) and the operator's grants from that screen (`AssistantGrantsScreen`: several assistants to several people at once, each with one budget, D57 to D63), with the repositories asking them through the module, at an effort the operator allows, or a person's own key directly (step 2) and what an app's warning before a private studyset goes names (`providerTermsOf`, `OwnAssistant.onThisMachine`, step 3), About with the instance's source link (`showAppAbout`), people's files on the device and the module's store (`FileCache`, `FileServerGateway`, `SealedFile`, `HLKF`, the format every app seals a file in) and a file opened in the system's app (`FileOpener`, `SystemFileOpener`: on Android through `HalkyonAppPlugin.java` and its own FileProvider class, `HalkyonFileProvider`; Halkyon's ADR 0043), the device's own speech recognition (`SpeechInput`, `DeviceSpeechInput`: on Android the voice input app handling `RECOGNIZE_SPEECH` where one is installed, `VoiceInputApp` on each app's `VoiceInput.kt`, `app.halkyon/voice_input`, FUTO Voice Input for one, which records itself and returns text, else `speech_to_text`; `FakeSpeechInput` in `testing.dart`; none on Linux; Meditate's ADR 0038, D18) and dictation into a text at its cursor, listening again after each pause (`Dictation`, Meditate's ADR 0038, D15; an app that listens declares the microphone and its usage texts itself), what's new after an update (`WhatsNew`, `WhatsNewOnUpdate`, `WhatsNewTile`, from the app's `assets/whats_new/`, Train's ADR 0030, D61), the device's appearance and language, chosen in Settings and applied before anyone signs in (`Look`, `FileLookStore`, `LookCubit`, `DeviceLookCard`; Learn keeps them among its own device settings and shows `LookSettingsCard`; ADR 0014's amendment of 4 Nov 2026), the language every app picks (`resolveAppLocale`) and countries' and subdivisions' names (Unicode CLDR's, bundled), with texts that name the app.
   - `halkyon_editor` (ADR 0033, D37, 6 Nov 2026): rich text, the general parts of Learn's editor on our fork of `appflowy_editor`: `RichContent` (the editor's document JSON, which Learn and Todo store), `RichContentEditor`, `RichContentView`, the format bar, links, the clipboard keeping formats, superscript and subscript, and `EditorBlocks`, through which an app adds blocks, marks within lines and their buttons (`basicBlocks` without any), with texts of its own (English, German). What is Learn's alone stays in Learn on top of it.
   - `halkyon_core` (low-level primitives in pure Dart: ids, a logger, `SerialQueue`, `CoalescingRunner`) and `halkyon_interop` (contracts between apps, pure Dart).