refactor(chain): 处理链功能域 mixin 化，清理未使用导入并根治兼容层循环导入

- ChainBase 拆分为 RecognitionMixin/MessageProcessingMixin/NotificationMixin
- TransferChain 拆分为 7 个功能 mixin（_mixins.py），SubscribeChain 音乐订阅域拆出 _music.py
- 斜杠命令交互四件套收敛为 InteractionChainMixin 委托，会话管理器移至 application 层，chain 层不再 re-export
- 模块基础类收敛到 app/modules/_base（notification/mediaserver 语义重命名）
- 清理 app/chain/__init__.py 24 个未使用导入，修正 49 处测试 patch 目标到实际命名空间
- 兼容层 legacy 符号不再并入 __all__，根治 schemas 初始化反向拉起 application.transfer 的循环导入
- 修复 bangumi 集数为字符串时 set_bangumi_info 抛 TypeError
- 新增重复代码等架构门禁测试；capability 清单校验排除下划线内部目录

tests/test_transfer_movie_collection.py

@@ -28,8 +28,8 @@ def _make_chain() -> TransferChain:
         (fileitem, False)
     ]
     chain._TransferChain__put_to_jobview = lambda task: True
-    chain._TransferChain__register_scrape_batch_task = lambda task: None
-    chain._TransferChain__close_scrape_batch = lambda batch_id: None
+    chain._register_scrape_batch_task = lambda task: None
+    chain._close_scrape_batch = lambda batch_id: None
     return chain
 
 
@@ -107,6 +107,7 @@ def test_conflicting_download_history_recognizes_movie_by_file_meta(monkeypatch)
         "app.chain.transfer.TransferHistoryOper",
         lambda: SimpleNamespace(get_by_type_tmdbid=lambda **kwargs: None),
     )
+    monkeypatch.setattr("app.chain._mixins.TransferHistoryOper", lambda: SimpleNamespace(get_by_type_tmdbid=lambda **kwargs: None))
     monkeypatch.setattr(
         "app.chain.transfer.MediaChain",
         lambda: SimpleNamespace(
@@ -117,6 +118,13 @@ def test_conflicting_download_history_recognizes_movie_by_file_meta(monkeypatch)
             supplement_tmdb_info=lambda media, _meta: media,
         ),
     )
+    monkeypatch.setattr("app.chain._mixins.MediaChain", lambda: SimpleNamespace(
+            recognize_media=lambda **kwargs: pytest.fail("不应按合集历史 ID 识别"),
+            recognize_by_meta=lambda meta, obtain_images: (
+                recognized_meta.append(meta) or fallback_media
+            ),
+            supplement_tmdb_info=lambda media, _meta: media,
+        ))
     task = TransferTask(
         fileitem=FileItem(
             storage="local",
@@ -182,12 +190,16 @@ def test_movie_collection_conflict_only_drops_automatic_media(
         "app.chain.transfer.TransferHistoryOper",
         lambda: SimpleNamespace(get_by_src=lambda src, storage=None: None),
     )
+    monkeypatch.setattr("app.chain._mixins.TransferHistoryOper", lambda: SimpleNamespace(get_by_src=lambda src, storage=None: None))
     monkeypatch.setattr("app.chain.transfer.DownloadHistoryOper", lambda: history_oper)
+    monkeypatch.setattr("app.chain._mixins.DownloadHistoryOper", lambda: history_oper)
     monkeypatch.setattr(
         "app.chain.transfer.SystemConfigOper",
         lambda: SimpleNamespace(get=lambda key: None),
     )
+    monkeypatch.setattr("app.chain._mixins.SystemConfigOper", lambda: SimpleNamespace(get=lambda key: None))
     monkeypatch.setattr("app.chain.transfer.StorageChain", lambda: SimpleNamespace())
+    monkeypatch.setattr("app.chain._mixins.StorageChain", lambda: SimpleNamespace())
     monkeypatch.setattr("app.chain.transfer.MetaInfoPath", lambda *args, **kwargs: file_meta)
 
     # 用真 MediaInfo 而非 SimpleNamespace：它会被装进 TransferTask.mediainfo，