feat(p1): add daily snapshots and fan tracking

Implement P1: seven-metric view snapshots, relation/stat fan snapshots, failure placeholders, resume by daily completion markers, Cookie login warnings, daily SQLite backups, Shanghai daily scheduling, and CLI snapshot/auth-check commands. Real read-only run captured 795/795 videos successfully over 1124s with a verified daily backup.

reaction_board/database.py

@@ -7,6 +7,7 @@
 
 from __future__ import annotations
 
+import os
 import queue
 import sqlite3
 import threading
@@ -165,6 +166,25 @@ class Database:
         finally:
             conn.close()
 
+    def backup_to(self, target: str | Path) -> Path:
+        """用 SQLite backup API 生成一致性备份；先写临时文件再原子替换。"""
+        target = Path(target)
+        target.parent.mkdir(parents=True, exist_ok=True)
+        tmp = target.with_name(target.name + ".tmp")
+        if tmp.exists():
+            tmp.unlink()
+
+        def run(conn: sqlite3.Connection) -> None:
+            dst = sqlite3.connect(tmp)
+            try:
+                conn.backup(dst)
+            finally:
+                dst.close()
+
+        self.write(run)
+        os.replace(tmp, target)
+        return target
+
     def close(self) -> None:
         if self._closed:
             return