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>

apps/learn/halkyon_learn_server/lib/src/auth/learn_base.dart

@@ -2,6 +2,7 @@ import 'package:halkyon_base_server/base.dart';
 import 'package:halkyon_learn_server/src/asset/learn_files.dart';
 import 'package:halkyon_learn_server/src/auth/account.dart';
 import 'package:halkyon_learn_server/src/instance/app_compatibility.dart';
+import 'package:halkyon_learn_server/src/shared_space/sync_space_grants.dart';
 
 /// Where this project's source is published, which the instance names
 /// unless its operator sets `SOURCE_URL`. Both licences require offering it
@@ -19,7 +20,9 @@ const upstreamSourceCodeUrl =
 /// person who goes ([deleteLearnData]), profile pages name the app at their
 /// foot, and it holds the instance's register with [accounts]' members
 /// ([learnAccounts]), and keeps people's files, which its asset rows name
-/// ([learnFiles], Halkyon's ADR 0043). The server sets it up before it starts (`server.dart`),
+/// ([learnFiles], Halkyon's ADR 0043), and the spaces people share on its
+/// sync engine ([SyncSpaceGrants], ADR 0044). The server sets it up before
+/// it starts (`server.dart`),
 /// and so do tests that delete people or read profiles.
 HalkyonBase learnBase({HoldsRegister accounts = const HoldsRegister()}) =>
     HalkyonBase(
@@ -37,6 +40,7 @@ HalkyonBase learnBase({HoldsRegister accounts = const HoldsRegister()}) =>
       privacyPagePoints: _privacyPagePoints,
       accounts: accounts,
       files: learnFiles,
+      spaces: SyncSpaceGrants(),
     );
 
 /// Learn's part in the instance's accounts (ADR 0016), from the operator's

apps/learn/halkyon_learn_server/lib/src/shared_space/sync_space_grants.dart

@@ -0,0 +1,57 @@
+import 'package:halkyon_base_server/base.dart';
+import 'package:serverpod/serverpod.dart';
+import 'package:serverpod_offline_sync_server/serverpod_offline_sync_server.dart';
+
+/// The sync engine's spaces as halkyon_base's shared spaces keep them in
+/// step (ADR 0044, D15): an Owner's and an Editor's grant writes, a
+/// Viewer's reads alone.
+final class SyncSpaceGrants() implements SpaceGrants {
+  @override
+  Future<UuidValue> create(
+    Session session, {
+    required UuidValue owner,
+    required Transaction transaction,
+  }) => OfflineSyncSpaces(session).createFor(owner, transaction: transaction);
+
+  @override
+  Future<void> grant(
+    Session session,
+    UuidValue space,
+    UuidValue person, {
+    required bool writes,
+    required Transaction transaction,
+  }) => OfflineSyncSpaces(session).grant(
+    space: space,
+    user: person,
+    role: writes
+        ? OfflineSyncSpaceRole.readWrite
+        : OfflineSyncSpaceRole.readOnly,
+    transaction: transaction,
+  );
+
+  @override
+  Future<void> revoke(
+    Session session,
+    UuidValue space,
+    UuidValue person, {
+    required Transaction transaction,
+  }) => OfflineSyncSpaces(
+    session,
+  ).revoke(space: space, user: person, transaction: transaction);
+
+  @override
+  Future<void> delete(
+    Session session,
+    UuidValue space, {
+    required Transaction transaction,
+  }) async {
+    // Every synced row of the space goes with it, as its tables' spaceId
+    // cascades.
+    // DELETE FROM "offline_sync_spaces" WHERE "uuidSpaceId" = $1
+    await OfflineSyncSpace.db.deleteWhere(
+      session,
+      where: (t) => t.uuidSpaceId.equals(space),
+      transaction: transaction,
+    );
+  }
+}