feat(02-01): Drift tables, DAOs, scheduling utility, domain models with tests

- Add Rooms, Tasks, TaskCompletions Drift tables with schema v2 migration
- Create RoomsDao with CRUD, watchAll, watchWithStats, cascade delete, reorder
- Create TasksDao with CRUD, watchInRoom (sorted by due), completeTask, overdue detection
- Implement calculateNextDueDate and catchUpToPresent pure scheduling functions
- Define IntervalType enum (8 types), EffortLevel enum, FrequencyInterval model
- Add formatRelativeDate German formatter and curatedRoomIcons icon list
- Enable PRAGMA foreign_keys in beforeOpen migration strategy
- All 30 unit tests passing (17 scheduling + 6 rooms DAO + 7 tasks DAO)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

test/core/database/database_test.dart

@@ -18,8 +18,8 @@ void main() {
       expect(db, isNotNull);
     });
 
-    test('has schemaVersion 1', () {
-      expect(db.schemaVersion, equals(1));
+    test('has schemaVersion 2', () {
+      expect(db.schemaVersion, equals(2));
     });
 
     test('can be closed without error', () async {