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/core/database/database.dart

@@ -35,6 +35,8 @@ class Tasks extends Table {
   DateTimeColumn get nextDueDate => dateTime()();
   DateTimeColumn get createdAt =>
       dateTime().clientDefault(() => DateTime.now())();
+  BoolColumn get isActive =>
+      boolean().withDefault(const Constant(true))();
 }
 
 /// TaskCompletions table: records when a task was completed.
@@ -53,7 +55,7 @@ class AppDatabase extends _$AppDatabase {
       : super(executor ?? _openConnection());
 
   @override
-  int get schemaVersion => 2;
+  int get schemaVersion => 3;
 
   @override
   MigrationStrategy get migration {
@@ -67,6 +69,9 @@ class AppDatabase extends _$AppDatabase {
           await m.createTable(tasks);
           await m.createTable(taskCompletions);
         }
+        if (from < 3) {
+          await m.addColumn(tasks, tasks.isActive);
+        }
       },
       beforeOpen: (details) async {
         await customStatement('PRAGMA foreign_keys = ON');