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

DEVELOPMENT.md

@@ -466,6 +466,13 @@ P1 实现说明：
 - Cookie 登录态通过 `x/web-interface/nav` 检查，失效只告警，不阻塞匿名 `view`；
 - Telegram 摘要推迟到 P3，P1 先写结构化 JSON 日志和 `kv` 状态。
 
+调度避让与补跑：
+
+- 全站 `snapshot` **不因**其它任务 running 而 skip；到期/强制必须执行；
+- 上海时区 **23:00–01:00** 为全站快照窗：`discovery` / `snapshot_recent` / `candidate_cleanup` 不执行，延后到 01:00 后；`fans` 短耗时，不避开任何任务；
+- 任务 skip（含静默窗延后）写入 `ok=false` + `skipped=true`，**不得**记成成功或推进「当日已完成」；
+- skip / fail 后短间隔补跑（约 5 分钟）；日更成功才推到次日 `snapshot_at` / `seed_at`。
+
 ---
 
 ## 8. Telegram 与入库操作