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.

reaction_board/web/queries.py

@@ -1982,6 +1982,7 @@ class DashboardQueries:
                 "snapshot_recent": int(interval_next_run(now, float(recent_interval))),
             }
             retry_seconds = 300
+            seed_retry_seconds = 600
             for name, key, at in (
                 ("fans", "fans", cfg.schedule.snapshot_at),
                 ("snapshot", "snapshot", cfg.schedule.snapshot_at),
@@ -2015,7 +2016,12 @@ class DashboardQueries:
                 else:
                     if skipped or failed:
                         # 日更任务失败/skip：短间隔补跑，绝不按成功推到次日。
-                        next_runs[name] = int(finished_at + retry_seconds)
+                        # snapshot_seed：全站快照占用 / no_source 时 10 分钟补跑。
+                        gap = (
+                            seed_retry_seconds if name == "snapshot_seed"
+                            else retry_seconds
+                        )
+                        next_runs[name] = int(finished_at + gap)
                     else:
                         next_runs[name] = int(next_daily_run(finished_at, at))
             errors = [