feat(todo): the sync engine and Todo's first models (ADR 0033, D5, D6)

Todo's server syncs with serverpod_offline_sync as Learn's and Train's
do, and holds two synced tables, every payload sealed on the devices:
content_key (the person's data key, sealed to the account) and project
(the Inbox or one of the person's own, changed when edited). A person's
space goes with them, with everything in it.

The migration only adds: the two tables, each indexed on spaceId and
cascading with its space (ON DELETE CASCADE to offline_sync_spaces), the
sync engine's own tables (crdt_*, offline_sync_*) with their foreign
keys, the gen_random_uuid_v7() function their ids default to, and the
modules' version rows. Nothing is dropped or changed.

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/halkyon_todo_client/lib/migrations/migration_registry.dart

@@ -0,0 +1,19 @@
+/// AUTOGENERATED BY SERVERPOD create-migration - DO NOT EDIT BY HAND
+/// Merging: resolve registry conflicts the same as migration_registry.txt.
+library;
+
+import 'package:serverpod_database/serverpod_database.dart';
+
+part '20261101000019144-projects/migration.dart';
+
+/// Migration registry for the client-side database.
+class MigrationRegistry {
+  /// Ordered list of all client-side database migrations.
+  /// New migrations are always appended at the end.
+  static final List<MigrationVersionSql> migrations = [
+    _Migration20261101000019144_projects(),
+  ];
+
+  /// List of all client-side database migration versions.
+  static List<String> get versions => migrations.map((m) => m.version).toList();
+}