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>

lib/features/tasks/domain/effort_level.dart

@@ -0,0 +1,23 @@
+/// Effort level for tasks.
+///
+/// IMPORTANT: Never reorder or remove values - intEnum stores the .index.
+/// Always add new values at the END.
+enum EffortLevel {
+  low, // 0
+  medium, // 1
+  high, // 2
+}
+
+/// German display labels for effort levels.
+extension EffortLevelLabel on EffortLevel {
+  String label() {
+    switch (this) {
+      case EffortLevel.low:
+        return 'Gering';
+      case EffortLevel.medium:
+        return 'Mittel';
+      case EffortLevel.high:
+        return 'Hoch';
+    }
+  }
+}