fix(cron): 修复固定间隔任务因 store 并发替换导致的重复执行

_on_timer 中 await _execute_job 让出控制权期间，前端轮询触发的
list_jobs 调用 _load_store 从磁盘重新加载覆盖 self._store，
已执行任务的状态被旧值回退，导致再次触发。
引入 _timer_active 标志位，在任务执行期间阻止并发 _load_store
替换 store。同时修复 store 为空时未重新 arm timer 的问题。

Made-with: Cursor

nanobot/cron/service.py

@@ -80,6 +80,7 @@ class CronService:
         self._store: CronStore | None = None
         self._timer_task: asyncio.Task | None = None
         self._running = False
+        self._timer_active = False
         self.max_sleep_ms = max_sleep_ms
 
     def _load_jobs(self) -> tuple[list[CronJob], int]:
@@ -171,7 +172,11 @@ class CronService:
     def _load_store(self) -> CronStore:
         """Load jobs from disk. Reloads automatically if file was modified externally.
         - Reload every time because it needs to merge operations on the jobs object from other instances.
+        - During _on_timer execution, return the existing store to prevent concurrent
+          _load_store calls (e.g. from list_jobs polling) from replacing it mid-execution.
         """
+        if self._timer_active and self._store:
+            return self._store
         jobs, version = self._load_jobs()
         self._store = CronStore(version=version, jobs=jobs)
         self._merge_action()
@@ -290,8 +295,11 @@ class CronService:
         """Handle timer tick - run due jobs."""
         self._load_store()
         if not self._store:
+            self._arm_timer()
             return
 
+        self._timer_active = True
+        try:
             now = _now_ms()
             due_jobs = [
                 j for j in self._store.jobs
@@ -302,6 +310,8 @@ class CronService:
                 await self._execute_job(job)
 
             self._save_store()
+        finally:
+            self._timer_active = False
         self._arm_timer()
 
     async def _execute_job(self, job: CronJob) -> None: