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/services/snapshot.py

@@ -51,6 +51,7 @@ class SnapshotSummary:
     completed: bool = False
     already_completed: bool = False
     finished_at: int = 0
+    reason: str | None = None
     errors: list[str] = field(default_factory=list)
 
     def as_dict(self) -> dict[str, Any]:
@@ -73,6 +74,7 @@ class SnapshotSummary:
             "backup_error": self.backup_error,
             "completed": self.completed,
             "already_completed": self.already_completed,
+            "reason": self.reason,
             "errors": self.errors,
         }
 
@@ -543,7 +545,8 @@ class SnapshotService:
                 stat_date=stat_date, reason="no_source", source=source,
             )
             summary.finished_at = self._now()
-            summary.completed = True
+            summary.completed = False
+            summary.reason = "no_source"
             return summary
         if self.repo.kv_get(self.completed_key(stat_date)) == "1":
             self._log(
@@ -553,6 +556,7 @@ class SnapshotService:
             summary.finished_at = self._now()
             summary.completed = True
             summary.already_completed = True
+            summary.reason = "already_completed"
             return summary
         if self.repo.kv_get(self.seeded_key(stat_date)) == "1":
             self._log(
@@ -562,6 +566,7 @@ class SnapshotService:
             summary.finished_at = self._now()
             summary.completed = True
             summary.already_completed = True
+            summary.reason = "already_seeded"
             return summary
 
         copied = self.repo.copy_ok_snapshots(
@@ -583,6 +588,7 @@ class SnapshotService:
                 self.repo.bump_cache_version("fans")
         summary.finished_at = self._now()
         summary.completed = True  # seed 任务本身成功；不等于快照落盘完成
+        summary.reason = "seeded"
         self._log(
             "info", "snapshot.seed_done",
             stat_date=stat_date, source=source,