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

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+/// 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';
+    }
+  }
+}

lib/features/tasks/domain/frequency.dart

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+/// 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),
+  ];
+}

lib/features/tasks/domain/relative_date.dart

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+/// Format a due date relative to today in German.
+///
+/// Returns labels like "Heute", "Morgen", "in X Tagen",
+/// "Uberfaellig seit 1 Tag", "Uberfaellig seit X Tagen".
+///
+/// Both [dueDate] and [today] are compared as date-only (ignoring time).
+String formatRelativeDate(DateTime dueDate, DateTime today) {
+  final diff =
+      DateTime(dueDate.year, dueDate.month, dueDate.day)
+          .difference(DateTime(today.year, today.month, today.day))
+          .inDays;
+
+  if (diff == 0) return 'Heute';
+  if (diff == 1) return 'Morgen';
+  if (diff > 1) return 'in $diff Tagen';
+  if (diff == -1) return 'Uberfaellig seit 1 Tag';
+  return 'Uberfaellig seit ${diff.abs()} Tagen';
+}

lib/features/tasks/domain/scheduling.dart

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+import 'frequency.dart';
+
+/// Calculate the next due date from the current due date and interval config.
+///
+/// For calendar-anchored intervals, [anchorDay] is the original day-of-month.
+/// Day-count intervals use pure arithmetic (Duration).
+/// Calendar-anchored intervals use month arithmetic with clamping.
+DateTime calculateNextDueDate({
+  required DateTime currentDueDate,
+  required IntervalType intervalType,
+  required int intervalDays,
+  int? anchorDay,
+}) {
+  switch (intervalType) {
+    // Day-count: pure arithmetic
+    case IntervalType.daily:
+      return currentDueDate.add(const Duration(days: 1));
+    case IntervalType.everyNDays:
+      return currentDueDate.add(Duration(days: intervalDays));
+    case IntervalType.weekly:
+      return currentDueDate.add(const Duration(days: 7));
+    case IntervalType.biweekly:
+      return currentDueDate.add(const Duration(days: 14));
+    // Calendar-anchored: month arithmetic with clamping
+    case IntervalType.monthly:
+      return _addMonths(currentDueDate, 1, anchorDay);
+    case IntervalType.everyNMonths:
+      return _addMonths(currentDueDate, intervalDays, anchorDay);
+    case IntervalType.quarterly:
+      return _addMonths(currentDueDate, 3, anchorDay);
+    case IntervalType.yearly:
+      return _addMonths(currentDueDate, 12, anchorDay);
+  }
+}
+
+/// Add months with day-of-month clamping.
+/// [anchorDay] remembers the original day for correct clamping.
+DateTime _addMonths(DateTime date, int months, int? anchorDay) {
+  final targetMonth = date.month + months;
+  final targetYear = date.year + (targetMonth - 1) ~/ 12;
+  final normalizedMonth = ((targetMonth - 1) % 12) + 1;
+  final day = anchorDay ?? date.day;
+  // Last day of target month: day 0 of next month
+  final lastDay = DateTime(targetYear, normalizedMonth + 1, 0).day;
+  final clampedDay = day > lastDay ? lastDay : day;
+  return DateTime(targetYear, normalizedMonth, clampedDay);
+}
+
+/// Catch-up: if next due is in the past, keep adding intervals until future/today.
+DateTime catchUpToPresent({
+  required DateTime nextDue,
+  required DateTime today,
+  required IntervalType intervalType,
+  required int intervalDays,
+  int? anchorDay,
+}) {
+  while (nextDue.isBefore(today)) {
+    nextDue = calculateNextDueDate(
+      currentDueDate: nextDue,
+      intervalType: intervalType,
+      intervalDays: intervalDays,
+      anchorDay: anchorDay,
+    );
+  }
+  return nextDue;
+}