feat(06-01): add watchCompletionsForTask DAO method and history localization strings

- Add watchCompletionsForTask(taskId) to TasksDao: Stream<List<TaskCompletion>> sorted newest first
- Regenerate tasks_dao.g.dart with build_runner
- Add taskHistoryTitle, taskHistoryEmpty, taskHistoryCount to app_de.arb
- Regenerate app_localizations.dart and app_localizations_de.dart
- All 5 new DAO tests pass, zero analyze issues

lib/features/tasks/data/tasks_dao.dart

@@ -81,6 +81,14 @@ class TasksDao extends DatabaseAccessor<AppDatabase> with _$TasksDaoMixin {
     });
   }
 
+  /// Watch all completions for a task, newest first.
+  Stream<List<TaskCompletion>> watchCompletionsForTask(int taskId) {
+    return (select(taskCompletions)
+          ..where((c) => c.taskId.equals(taskId))
+          ..orderBy([(c) => OrderingTerm.desc(c.completedAt)]))
+        .watch();
+  }
+
   /// Count overdue tasks in a room (nextDueDate before today).
   Future<int> getOverdueTaskCount(int roomId, {DateTime? today}) async {
     final now = today ?? DateTime.now();

lib/l10n/app_de.arb

@@ -107,5 +107,13 @@
       "overdue": { "type": "int" }
     }
   },
-  "calendarTodayButton": "Heute"
+  "calendarTodayButton": "Heute",
+  "taskHistoryTitle": "Verlauf",
+  "taskHistoryEmpty": "Noch nie erledigt",
+  "taskHistoryCount": "{count} Mal erledigt",
+  "@taskHistoryCount": {
+    "placeholders": {
+      "count": { "type": "int" }
+    }
+  }
 }

lib/l10n/app_localizations.dart

@@ -519,6 +519,24 @@ abstract class AppLocalizations {
   /// In de, this message translates to:
   /// **'Heute'**
   String get calendarTodayButton;
+
+  /// No description provided for @taskHistoryTitle.
+  ///
+  /// In de, this message translates to:
+  /// **'Verlauf'**
+  String get taskHistoryTitle;
+
+  /// No description provided for @taskHistoryEmpty.
+  ///
+  /// In de, this message translates to:
+  /// **'Noch nie erledigt'**
+  String get taskHistoryEmpty;
+
+  /// No description provided for @taskHistoryCount.
+  ///
+  /// In de, this message translates to:
+  /// **'{count} Mal erledigt'**
+  String taskHistoryCount(int count);
 }
 
 class _AppLocalizationsDelegate

lib/l10n/app_localizations_de.dart

@@ -239,4 +239,15 @@ class AppLocalizationsDe extends AppLocalizations {
 
   @override
   String get calendarTodayButton => 'Heute';
+
+  @override
+  String get taskHistoryTitle => 'Verlauf';
+
+  @override
+  String get taskHistoryEmpty => 'Noch nie erledigt';
+
+  @override
+  String taskHistoryCount(int count) {
+    return '$count Mal erledigt';
+  }
 }