perf(cache): enforce cover byte budget during foreground use

lib/utils/periodic_async_task.dart

@@ -0,0 +1,48 @@
+import 'dart:async';
+
+/// Foreground maintenance that waits between completed runs and can be paused
+/// safely even while a run is awaiting I/O.
+class PeriodicAsyncTask {
+  PeriodicAsyncTask({required this.interval, required this.run, this.onError});
+  final Duration interval;
+  final Future<void> Function() run;
+  final void Function(Object, StackTrace)? onError;
+  Timer? _timer;
+  bool _enabled = false;
+  int _generation = 0;
+  Future<void> _pending = Future<void>.value();
+
+  void start({required Duration delay}) {
+    if (_enabled) return;
+    _enabled = true;
+    _schedule(delay, _generation);
+  }
+
+  void stop() {
+    _enabled = false;
+    _generation++;
+    _timer?.cancel();
+    _timer = null;
+  }
+
+  void _schedule(Duration delay, int generation) {
+    _timer = Timer(delay, () async {
+      final operation = _pending.then((_) async {
+        if (!_enabled || generation != _generation) return;
+        try {
+          await run();
+        } catch (error, stack) {
+          onError?.call(error, stack);
+        }
+      });
+      _pending = operation;
+      try {
+        await operation;
+      } finally {
+        if (_enabled && generation == _generation) {
+          _schedule(interval, generation);
+        }
+      }
+    });
+  }
+}