fix(scheduler): drop cross-task mutex, quiet window + retry

Incident: evening full snapshot could be recorded as ok with
already_completed/elapsed=0 after mutual skip with hourly sync, then
next_run jumped to the next day.

- Full-site snapshot never skips because another task is running
- 23:00-01:00 Asia/Shanghai: defer discovery / snapshot_recent /
  candidate_cleanup to after 01:00; fans never defers
- Skip records ok=false + skipped=true (not success / not completed)
- Skip/fail reschedule soon (~5m); daily success still next snapshot_at
- TaskControl return value drives next_run; system UI shows skip fields

project.md

@@ -800,7 +800,7 @@ secrets.example.json            凭据样例（auth 回写目标）
 **进程结构**
 
 ```
-scheduler 线程：discovery(每 60min) / snapshot_seed(每日 00:00) / snapshot+fans(每日 23:15) / snapshot_recent(小时) / candidate_cleanup(每日)
+scheduler 线程：discovery(每 discover_minutes，避开 23:00–01:00) / snapshot_seed(每日 00:00) / snapshot(每日 23:15，不因其它任务 skip) / fans(每日 23:15，不避开) / snapshot_recent(小时，避开 23:00–01:00) / candidate_cleanup(每日，避开 23:00–01:00)
 web 线程：ThreadingHTTPServer :8080
 telegram：getUpdates long polling（回调 + 私聊文本入库）
 storage：SQLite WAL，单写线程