feat(08-01): add isActive column, migration v3, softDeleteTask and getCompletionCount

- Add isActive BoolColumn (default true) to Tasks table
- Bump schema version from 2 to 3 with addColumn migration
- Filter watchTasksInRoom to isActive=true only
- Filter getOverdueTaskCount to isActive=true only
- Add softDeleteTask(taskId) - sets isActive=false without removing data
- Add getCompletionCount(taskId) - counts TaskCompletions for a task

lib/features/tasks/data/tasks_dao.dart

@@ -10,9 +10,10 @@ class TasksDao extends DatabaseAccessor<AppDatabase> with _$TasksDaoMixin {
   TasksDao(super.attachedDatabase);
 
   /// Watch tasks in a room sorted by nextDueDate ascending.
+  /// Only returns active tasks (isActive = true).
   Stream<List<Task>> watchTasksInRoom(int roomId) {
     return (select(tasks)
-          ..where((t) => t.roomId.equals(roomId))
+          ..where((t) => t.roomId.equals(roomId) & t.isActive.equals(true))
           ..orderBy([(t) => OrderingTerm.asc(t.nextDueDate)]))
         .watch();
   }
@@ -90,12 +91,13 @@ class TasksDao extends DatabaseAccessor<AppDatabase> with _$TasksDaoMixin {
   }
 
   /// Count overdue tasks in a room (nextDueDate before today).
+  /// Only counts active tasks (isActive = true).
   Future<int> getOverdueTaskCount(int roomId, {DateTime? today}) async {
     final now = today ?? DateTime.now();
     final todayDateOnly = DateTime(now.year, now.month, now.day);
 
     final taskList = await (select(tasks)
-          ..where((t) => t.roomId.equals(roomId)))
+          ..where((t) => t.roomId.equals(roomId) & t.isActive.equals(true)))
         .get();
 
     return taskList.where((task) {
@@ -107,4 +109,21 @@ class TasksDao extends DatabaseAccessor<AppDatabase> with _$TasksDaoMixin {
       return dueDate.isBefore(todayDateOnly);
     }).length;
   }
+
+  /// Soft-delete a task by setting isActive to false.
+  /// The task and its completions remain in the database.
+  Future<void> softDeleteTask(int taskId) {
+    return (update(tasks)..where((t) => t.id.equals(taskId)))
+        .write(const TasksCompanion(isActive: Value(false)));
+  }
+
+  /// Count completions for a task.
+  Future<int> getCompletionCount(int taskId) async {
+    final count = taskCompletions.id.count();
+    final query = selectOnly(taskCompletions)
+      ..addColumns([count])
+      ..where(taskCompletions.taskId.equals(taskId));
+    final result = await query.getSingle();
+    return result.read(count) ?? 0;
+  }
 }