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_overwrite_declined.py

@@ -53,7 +53,7 @@ def test_overwrite_declined_false_when_flag_not_set():
     transferinfo = TransferInfo(success=False, overwrite_skipped=False)
     transferhis = make_history_oper(raise_on_query=True)
 
-    result = TransferChain._TransferChain__is_overwrite_declined(
+    result = TransferChain._is_overwrite_declined(
         task, transferinfo, transferhis
     )
 
@@ -67,7 +67,7 @@ def test_overwrite_declined_true_when_success_history_exists():
     transferinfo = TransferInfo(success=False, overwrite_skipped=True)
     transferhis = make_history_oper(history=success_history)
 
-    result = TransferChain._TransferChain__is_overwrite_declined(
+    result = TransferChain._is_overwrite_declined(
         task, transferinfo, transferhis
     )
 
@@ -80,7 +80,7 @@ def test_overwrite_declined_false_when_no_history():
     transferinfo = TransferInfo(success=False, overwrite_skipped=True)
     transferhis = make_history_oper(history=None)
 
-    result = TransferChain._TransferChain__is_overwrite_declined(
+    result = TransferChain._is_overwrite_declined(
         task, transferinfo, transferhis
     )
 
@@ -94,7 +94,7 @@ def test_overwrite_declined_false_when_only_failed_history():
     transferinfo = TransferInfo(success=False, overwrite_skipped=True)
     transferhis = make_history_oper(history=failed_history, success_history=None)
 
-    result = TransferChain._TransferChain__is_overwrite_declined(
+    result = TransferChain._is_overwrite_declined(
         task, transferinfo, transferhis
     )
 
@@ -107,7 +107,7 @@ def test_overwrite_declined_false_when_query_raises():
     transferinfo = TransferInfo(success=False, overwrite_skipped=True)
     transferhis = make_history_oper(raise_on_query=True)
 
-    result = TransferChain._TransferChain__is_overwrite_declined(
+    result = TransferChain._is_overwrite_declined(
         task, transferinfo, transferhis
     )