feat(scheduler): add snapshot-seed CLI and 10m seed deferral

Add `python app.py snapshot-seed [--date]` (offline clone, Debian /
docker compose exec friendly). When seed fires while full-site snapshot
is running, or last_completed is missing (no_source), record skip as
non-success and retry in 10 minutes until a real seed succeeds; fans
and quiet-window rules unchanged.

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 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)
+scheduler 线程：discovery(每 discover_minutes，避开 23:00–01:00) / snapshot_seed(每日 00:00；snapshot 占用或 no_source 则 10 分钟重试，CLI: snapshot-seed) / 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