feat(todo): sections, tasks and labels on the server (ADR 0033, D6)

Three more synced tables, each a sealed payload alone, as project's:
section, task and label; deleted with the person's space.

The migration only adds: the three tables (id uuid defaulting to
gen_random_uuid_v7(), spaceId, keyId, payload, nonce), each indexed on
spaceId and cascading with its space (ON DELETE CASCADE to
offline_sync_spaces), 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

@@ -5,6 +5,7 @@ library;
 import 'package:serverpod_database/serverpod_database.dart';
 
 part '20261101000019144-projects/migration.dart';
+part '20261101000604776-tasks/migration.dart';
 
 /// Migration registry for the client-side database.
 class MigrationRegistry {
@@ -12,6 +13,7 @@ class MigrationRegistry {
   /// New migrations are always appended at the end.
   static final List<MigrationVersionSql> migrations = [
     _Migration20261101000019144_projects(),
+    _Migration20261101000604776_tasks(),
   ];
 
   /// List of all client-side database migration versions.