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_mounted_disk_cleanup.py

@@ -31,10 +31,10 @@ def test_enabled_cleanup_skips_filesystem_detection():
     开关开启时应保持旧行为，且不产生额外文件系统检测。
     """
     with patch(
-            "app.chain.transfer.SystemUtils.is_network_filesystem"
+            "app.chain._mixins.SystemUtils.is_network_filesystem"
     ) as is_network_filesystem:
         should_delete = (
-            TransferChain._TransferChain__should_delete_empty_source_directories(
+            TransferChain._should_delete_empty_source_directories(
                 _make_task(),
                 True,
                 {},
@@ -50,11 +50,11 @@ def test_disabled_cleanup_keeps_mounted_local_source_directories():
     开关关闭时应保留网络或 FUSE 挂载的本地源目录。
     """
     with patch(
-            "app.chain.transfer.SystemUtils.is_network_filesystem",
+            "app.chain._mixins.SystemUtils.is_network_filesystem",
             return_value=True,
     ) as is_network_filesystem:
         should_delete = (
-            TransferChain._TransferChain__should_delete_empty_source_directories(
+            TransferChain._should_delete_empty_source_directories(
                 _make_task(),
                 False,
                 {},
@@ -72,11 +72,11 @@ def test_disabled_cleanup_still_deletes_ordinary_local_source_directories():
     开关关闭时普通本地文件系统仍应删除空目录。
     """
     with patch(
-            "app.chain.transfer.SystemUtils.is_network_filesystem",
+            "app.chain._mixins.SystemUtils.is_network_filesystem",
             return_value=False,
     ):
         should_delete = (
-            TransferChain._TransferChain__should_delete_empty_source_directories(
+            TransferChain._should_delete_empty_source_directories(
                 _make_task(download_path="/downloads"),
                 False,
                 {},
@@ -91,10 +91,10 @@ def test_disabled_cleanup_does_not_change_remote_storage_cleanup():
     开关关闭时非本地存储仍应执行原有空目录清理。
     """
     with patch(
-            "app.chain.transfer.SystemUtils.is_network_filesystem"
+            "app.chain._mixins.SystemUtils.is_network_filesystem"
     ) as is_network_filesystem:
         should_delete = (
-            TransferChain._TransferChain__should_delete_empty_source_directories(
+            TransferChain._should_delete_empty_source_directories(
                 _make_task(storage="alist", download_path="/downloads"),
                 False,
                 {},
@@ -111,12 +111,12 @@ def test_mounted_filesystem_detection_is_cached_by_source_directory():
     """
     mounted_filesystem_cache = {}
     with patch(
-            "app.chain.transfer.SystemUtils.is_network_filesystem",
+            "app.chain._mixins.SystemUtils.is_network_filesystem",
             return_value=True,
     ) as is_network_filesystem:
         for _ in range(2):
             should_delete = (
-                TransferChain._TransferChain__should_delete_empty_source_directories(
+                TransferChain._should_delete_empty_source_directories(
                     _make_task(),
                     False,
                     mounted_filesystem_cache,