调整图片下载逻辑，定时任务5分钟一次进行图片消息下载存档。

db/message_storage.py

@@ -257,3 +257,27 @@ class MessageStorageDB(BaseDBOperator):
         """
         
         return self.execute_query(sql, tuple(params)) or []
+
+    def get_pending_image_messages(self, minutes_ago: int = 10, limit: int = 50) -> List[Dict]:
+        """获取最近N分钟内未处理图片的消息（image_path IS NULL）
+        
+        Args:
+            minutes_ago: 查询最近多少分钟的消息，默认10分钟
+            limit: 每次最多处理多少条，默认50条
+            
+        Returns:
+            包含消息ID、群ID、消息XML等信息的列表
+        """
+        sql = """
+        SELECT message_id, group_id, message_xml, timestamp
+        FROM messages
+        WHERE message_type = '3'
+          AND image_path IS NULL
+          AND timestamp >= DATE_SUB(NOW(), INTERVAL %s MINUTE)
+          AND message_xml IS NOT NULL
+          AND message_xml != ''
+        ORDER BY timestamp ASC
+        LIMIT %s
+        """
+        params = (minutes_ago, limit)
+        return self.execute_query(sql, params) or []