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/frequency.dart

@@ -0,0 +1,62 @@
+/// Frequency interval types for recurring tasks.
+///
+/// IMPORTANT: Never reorder or remove values - intEnum stores the .index.
+/// Always add new values at the END.
+enum IntervalType {
+  daily, // 0
+  everyNDays, // 1
+  weekly, // 2
+  biweekly, // 3
+  monthly, // 4
+  everyNMonths, // 5
+  quarterly, // 6
+  yearly, // 7
+}
+
+/// A frequency interval combining a type with an optional multiplier.
+class FrequencyInterval {
+  final IntervalType intervalType;
+  final int days;
+
+  const FrequencyInterval({required this.intervalType, this.days = 1});
+
+  /// German display label for this interval.
+  String label() {
+    switch (intervalType) {
+      case IntervalType.daily:
+        return 'Taeglich';
+      case IntervalType.everyNDays:
+        if (days == 7) return 'Woechentlich';
+        if (days == 14) return 'Alle 2 Wochen';
+        return 'Alle $days Tage';
+      case IntervalType.weekly:
+        return 'Woechentlich';
+      case IntervalType.biweekly:
+        return 'Alle 2 Wochen';
+      case IntervalType.monthly:
+        return 'Monatlich';
+      case IntervalType.everyNMonths:
+        if (days == 3) return 'Vierteljaehrlich';
+        if (days == 6) return 'Halbjaehrlich';
+        return 'Alle $days Monate';
+      case IntervalType.quarterly:
+        return 'Vierteljaehrlich';
+      case IntervalType.yearly:
+        return 'Jaehrlich';
+    }
+  }
+
+  /// All preset frequency intervals per TASK-04.
+  static const List<FrequencyInterval> presets = [
+    FrequencyInterval(intervalType: IntervalType.daily),
+    FrequencyInterval(intervalType: IntervalType.everyNDays, days: 2),
+    FrequencyInterval(intervalType: IntervalType.everyNDays, days: 3),
+    FrequencyInterval(intervalType: IntervalType.weekly),
+    FrequencyInterval(intervalType: IntervalType.biweekly),
+    FrequencyInterval(intervalType: IntervalType.monthly),
+    FrequencyInterval(intervalType: IntervalType.everyNMonths, days: 2),
+    FrequencyInterval(intervalType: IntervalType.quarterly),
+    FrequencyInterval(intervalType: IntervalType.everyNMonths, days: 6),
+    FrequencyInterval(intervalType: IntervalType.yearly),
+  ];
+}