更新迁移脚本，支持断点续传和批量导入，更新数据库模型，允许群聊信息字段为可选（null=True）

src/common/database/database_model.py

@@ -44,9 +44,9 @@ class ChatStreams(BaseModel):
     #   platform: "qq"
     #   group_id: "941657197"
     #   group_name: "测试"
-    group_platform = TextField()
-    group_id = TextField()
-    group_name = TextField()
+    group_platform = TextField(null=True)  # 群聊信息可能不存在
+    group_id = TextField(null=True)
+    group_name = TextField(null=True)
 
     # last_active_time: 1746623771.4825106 (时间戳，精确到小数点后7位)
     last_active_time = DoubleField()