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）。

tests/test_episode_format_helper.py

@@ -722,7 +722,7 @@ def test_transfer_chain_recommend_episode_format_passes_helper_data(monkeypatch)
         lambda item: [sample],
     )
     monkeypatch.setattr(
-        "app.chain._mixins.EpisodeFormatRuleHelper.recommend",
+        "app.chain._transfer.EpisodeFormatRuleHelper.recommend",
         lambda self, rules, sample_files: (True, "", helper_data),
     )
 
@@ -777,7 +777,7 @@ def test_transfer_chain_recommend_episode_format_uses_selected_fileitems(monkeyp
         lambda: [],
     )
     monkeypatch.setattr(
-        "app.chain._mixins.EpisodeFormatRuleHelper.recommend",
+        "app.chain._transfer.EpisodeFormatRuleHelper.recommend",
         lambda self, rules, sample_files: (True, "", {
             **helper_data,
             "received_samples": [item.name for item in sample_files],