fix: VLM复核前将COS object key转为预签名URL

snapshot_url存储的是COS对象路径而非完整URL，
VLM无法直接访问，需先生成预签名URL。

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/services/notify_dispatch.py

@@ -47,8 +47,14 @@ async def process_alarm_notification(alarm_data: Dict):
         camera_name = alarm_data.get("camera_name", device_id)
         roi_name = alarm_data.get("scene_id", "")
 
+        # snapshot_url 可能是 COS object key，需转为可访问的预签名URL
+        vlm_snapshot_url = snapshot_url
+        if snapshot_url and not snapshot_url.startswith("http"):
+            from app.services.oss_storage import get_oss_storage
+            vlm_snapshot_url = get_oss_storage().get_presigned_url(snapshot_url)
+
         vlm_result = await vlm_service.verify_alarm(
-            snapshot_url=snapshot_url,
+            snapshot_url=vlm_snapshot_url,
             alarm_type=alarm_type,
             camera_name=camera_name,
             roi_name=roi_name,