feat: backfill_frames 支持压缩缓存回退 — 补齐全部历史事件帧图

原始视频被 Surveillance Station 保留策略删除后，回退到
/tmp/fam_compressed/task_{id}/ 的 480p 压缩副本抽帧。实测
event 18-27（08-14 晚间 10 事件）补 78 帧全部成功，至此
所有历史事件帧图齐全，UI 不再出现暂无帧图。

fam-core/tools/backfill_frames.py

@@ -81,7 +81,7 @@ def main():
     try:
         with conn.cursor() as cur:
             sql = """
-                SELECT me.event_id, me.event_start_time, pt.video_path,
+                SELECT me.event_id, me.task_id, me.event_start_time, pt.video_path,
                        (SELECT COUNT(*) FROM event_details ed
                          WHERE ed.event_id = me.event_id) AS detail_count
                 FROM monitor_events me
@@ -115,8 +115,16 @@ def main():
                 continue
 
             video = ev.get('video_path') or ''
+            source = '原始视频'
             if not video or not os.path.isfile(video):
-                print(f'[event {eid}] 视频不存在，跳过 {len(missing)} 帧: {video}')
+                # 原始视频被监控保留策略清理时，回退到 dispatcher 压缩缓存（480p 副本）
+                cached = os.path.join(
+                    f'/tmp/fam_compressed/task_{ev["task_id"]}', os.path.basename(video)) if video else ''
+                if cached and os.path.isfile(cached):
+                    video = cached
+                    source = '压缩缓存'
+            if not video or not os.path.isfile(video):
+                print(f'[event {eid}] 原始视频与压缩缓存均不存在，跳过 {len(missing)} 帧: {video}')
                 stat['skip_no_video'] += 1
                 continue
 
@@ -127,7 +135,7 @@ def main():
 
             os.makedirs(edir, exist_ok=True)
             stat['events'] += 1
-            print(f'[event {eid}] 补 {len(missing)}/{len(frames)} 帧 (视频 {os.path.basename(video)}, {dur:.0f}s)')
+            print(f'[event {eid}] 补 {len(missing)}/{len(frames)} 帧 · {source} (视频 {os.path.basename(video)}, {dur:.0f}s)')
 
             for f in missing:
                 out_path = os.path.join(edir, f'frame_{f["frame_index"]}.jpg')