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_custom_words.py

@@ -7,7 +7,7 @@
 """
 from types import SimpleNamespace
 
-import app.chain.transfer as transfer_module
+import app.chain._mixins as mixins_module
 from app.chain.transfer import TransferChain
 
 
@@ -26,7 +26,7 @@ def test_transfer_prefers_snapshot_over_live_lookup(monkeypatch):
             called["lookup"] = True
             return SimpleNamespace(custom_words="不应使用\n实时反查")
 
-    monkeypatch.setattr(transfer_module, "SubscribeChain", _GuardSubscribeChain)
+    monkeypatch.setattr(mixins_module, "SubscribeChain", _GuardSubscribeChain)
 
     history = _fake_history(
         custom_words="S04 => S01\n第 <> 集 >> EP+66",
@@ -46,7 +46,7 @@ def test_transfer_falls_back_to_live_lookup_without_snapshot(monkeypatch):
             assert source == "Subscribe|{...}"
             return SimpleNamespace(custom_words="A => B")
 
-    monkeypatch.setattr(transfer_module, "SubscribeChain", _FakeSubscribeChain)
+    monkeypatch.setattr(mixins_module, "SubscribeChain", _FakeSubscribeChain)
 
     history = _fake_history(custom_words=None, note={"source": "Subscribe|{...}"})
     result = TransferChain._get_subscribe_custom_words(history)
@@ -61,7 +61,7 @@ def test_transfer_returns_none_when_unavailable(monkeypatch):
         def get_subscribe_by_source(self, source):
             return None
 
-    monkeypatch.setattr(transfer_module, "SubscribeChain", _NoneSubscribeChain)
+    monkeypatch.setattr(mixins_module, "SubscribeChain", _NoneSubscribeChain)
 
     # 无下载记录
     assert TransferChain._get_subscribe_custom_words(None) is None