refactor(chain): _mixins.py 更名 _transfer.py，统一功能域 mixin 命名

TransferChain 的 7 个功能域 mixin 集合文件按域更名 _transfer.py，
与 _recognition/_messaging/_interaction/_music 的单域命名约定对齐；
同步更新 transfer.py 导入、9 个测试文件 patch 目标与架构文档。
另含 chain 基类导入排序优化与未使用导入清理（copy/datetime 已随
dispatch 方法迁入 _messaging.py）。

app/chain/__init__.py

@@ -1,25 +1,26 @@
 from __future__ import annotations
 
-import copy
 import inspect
 import pickle
 import traceback
 from abc import ABCMeta
 from collections.abc import Callable
-from datetime import datetime
 from pathlib import Path
 from typing import Optional, Any, Tuple, List, Set, Union, Dict
 
 from fastapi.concurrency import run_in_threadpool
 
-from app.runtime.cache import FileCache, AsyncFileCache
+from app.application.messaging.message import MessageHelper, MessageQueueManager
+from app.chain._messaging import MessageProcessingMixin, NotificationMixin
+from app.chain._recognition import RecognitionMixin
+from app.db.oper.message import MessageOper
 from app.domain.context import Context, MediaInfo, SubtitleInfo, TorrentInfo
-from app.runtime.events import EventManager
 from app.domain.meta.metabase import MetaBase
+from app.foundation.reflection import ObjectUtils
+from app.runtime.cache import FileCache, AsyncFileCache
+from app.runtime.events import EventManager
 from app.runtime.extensions.module_manager import ModuleManager
 from app.runtime.extensions.plugin_manager import PluginManager
-from app.db.oper.message import MessageOper
-from app.application.messaging.message import MessageHelper, MessageQueueManager
 from app.runtime.log import logger
 from app.schemas import (
     RateLimitExceededException,
@@ -41,9 +42,6 @@ from app.schemas.types import (
     MediaImageType,
     EventType,
 )
-from app.foundation.reflection import ObjectUtils
-from app.chain._messaging import MessageProcessingMixin, NotificationMixin
-from app.chain._recognition import RecognitionMixin
 
 
 class ChainBase(RecognitionMixin, MessageProcessingMixin, NotificationMixin,

app/chain/transfer.py

@@ -57,7 +57,7 @@ from app.schemas.types import (
 from app.runtime.reload import ConfigReloadMixin
 from app.application.transfer import (FailedRetryScheduler, JobManager,
                                       TransferQueue, TransferTask, job_lock)
-from app.chain._mixins import (EpisodeFormatMixin, FailedRetryMixin,
+from app.chain._transfer import (EpisodeFormatMixin, FailedRetryMixin,
                                FileFilterMixin, FileKeyMixin,
                                HistoryMatchMixin, ManualHistoryMixin,
                                ScrapeBatchMixin)