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_music_workflows.py

@@ -435,7 +435,7 @@ def test_media_chain_default_recognition_only_queries_musicbrainz(monkeypatch):
     recognize_source = Mock(return_value=expected)
     monkeypatch.setattr(chain, "recognize_music_from_source", recognize_source)
 
-    with patch("app.chain.MoviePilotServerHelper.report_recognize_share"):
+    with patch("app.chain._recognition.MoviePilotServerHelper.report_recognize_share"):
         result = chain.recognize_media(meta=meta)
 
     assert result is expected
@@ -484,7 +484,7 @@ def test_default_recognition_does_not_fallback_after_musicbrainz_miss(monkeypatc
     monkeypatch.setattr(chain, "recognize_music_from_source", recognize_source)
 
     with patch(
-        "app.chain.MoviePilotServerHelper.query_recognize_share",
+        "app.chain._recognition.MoviePilotServerHelper.query_recognize_share",
         return_value=None,
     ):
         assert chain.recognize_media(meta=meta) is None
@@ -506,7 +506,7 @@ def test_async_default_recognition_only_queries_musicbrainz(monkeypatch):
     monkeypatch.setattr(chain, "async_recognize_music_from_source", recognize_source)
 
     with patch(
-        "app.chain.MoviePilotServerHelper.async_report_recognize_share",
+        "app.chain._recognition.MoviePilotServerHelper.async_report_recognize_share",
         new=AsyncMock(),
     ):
         result = asyncio.run(chain.async_recognize_media(meta=meta))