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_episode_format_helper.py

@@ -708,21 +708,21 @@ def test_transfer_chain_recommend_episode_format_passes_helper_data(monkeypatch)
 
     monkeypatch.setattr(
         chain,
-        "_TransferChain__resolve_episode_format_directory",
+        "_resolve_episode_format_directory",
         lambda item: directory,
     )
     monkeypatch.setattr(
         chain,
-        "_TransferChain__get_episode_format_rules",
+        "_get_episode_format_rules",
         lambda: [],
     )
     monkeypatch.setattr(
         chain,
-        "_TransferChain__get_episode_format_sample_files",
+        "_get_episode_format_sample_files",
         lambda item: [sample],
     )
     monkeypatch.setattr(
-        "app.chain.transfer.EpisodeFormatRuleHelper.recommend",
+        "app.chain._mixins.EpisodeFormatRuleHelper.recommend",
         lambda self, rules, sample_files: (True, "", helper_data),
     )
 
@@ -773,11 +773,11 @@ def test_transfer_chain_recommend_episode_format_uses_selected_fileitems(monkeyp
 
     monkeypatch.setattr(
         chain,
-        "_TransferChain__get_episode_format_rules",
+        "_get_episode_format_rules",
         lambda: [],
     )
     monkeypatch.setattr(
-        "app.chain.transfer.EpisodeFormatRuleHelper.recommend",
+        "app.chain._mixins.EpisodeFormatRuleHelper.recommend",
         lambda self, rules, sample_files: (True, "", {
             **helper_data,
             "received_samples": [item.name for item in sample_files],
@@ -854,7 +854,7 @@ def test_transfer_chain_episode_format_samples_include_extra_files(monkeypatch):
     monkeypatch.setattr(chain, "_subtitle_exts", [".ass", ".ssa"], raising=False)
     monkeypatch.setattr(chain, "_audio_exts", [".mka", ".aac"], raising=False)
 
-    sample_files = TransferChain._TransferChain__get_episode_format_sample_files(
+    sample_files = TransferChain._get_episode_format_sample_files(
         chain,
         directory,
     )