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_subscribe.py

@@ -132,9 +132,9 @@ def test_music_subscribe_reuses_search_download_and_finish_flow():
     chain.filter_torrents = Mock(side_effect=lambda **kwargs: kwargs["torrent_list"])
 
     with patch.object(SubscribeChain, "_recognize_music_subscribe", return_value=target), \
-            patch("app.chain.subscribe.SearchChain", return_value=search_chain), \
-            patch("app.chain.subscribe.DownloadChain", return_value=download_chain), \
-            patch("app.chain.subscribe.SubscribeOper") as subscribe_oper:
+            patch("app.chain._music.SearchChain", return_value=search_chain), \
+            patch("app.chain._music.DownloadChain", return_value=download_chain), \
+            patch("app.chain._music.SubscribeOper") as subscribe_oper:
         subscribe_oper.return_value.get.return_value = subscribe
         chain._search_music_subscribe(subscribe)
 
@@ -239,8 +239,8 @@ def test_music_best_version_persists_downloaded_rule_priority():
     chain = SubscribeChain()
     chain.finish_subscribe_or_not = Mock()
 
-    with patch("app.chain.subscribe.DownloadChain", return_value=download_chain), \
-            patch("app.chain.subscribe.SubscribeOper", return_value=subscribe_oper):
+    with patch("app.chain._music.DownloadChain", return_value=download_chain), \
+            patch("app.chain._music.SubscribeOper", return_value=subscribe_oper):
         chain._download_music_subscribe(subscribe, _music_info(), [downloaded])
 
     subscribe_oper.update.assert_called_once_with(
@@ -441,8 +441,9 @@ def test_music_rss_match_reuses_cached_context_without_second_site_search():
     torrent_helper.filter_torrent.return_value = True
     with patch.object(SubscribeChain, "_recognize_music_subscribe", return_value=target), \
             patch("app.chain.subscribe.SubscribeOper", return_value=subscribe_oper), \
-            patch("app.chain.subscribe.TorrentHelper", return_value=torrent_helper), \
-            patch("app.chain.subscribe.DownloadChain", return_value=download_chain), \
+            patch("app.chain._music.SubscribeOper", return_value=subscribe_oper), \
+            patch("app.chain._music.TorrentHelper", return_value=torrent_helper), \
+            patch("app.chain._music.DownloadChain", return_value=download_chain), \
             patch("app.chain.subscribe.SearchChain") as search_chain, \
             patch("app.chain.subscribe.MediaChain") as media_chain:
         chain.match({"music.example": [source_context]})
@@ -519,7 +520,7 @@ def test_legacy_music_without_identity_uses_recording_recognition_boundary():
     media_chain = Mock()
     media_chain.recognize_media.return_value = recording
 
-    with patch("app.chain.subscribe.MediaChain", return_value=media_chain):
+    with patch("app.chain._music.MediaChain", return_value=media_chain):
         restored = SubscribeChain._recognize_music_subscribe(subscribe)
 
     assert restored is recording
@@ -637,7 +638,7 @@ def test_recording_target_sync_clears_stale_album_track_count():
     subscribe = _subscribe(total_tracks=11)
     subscribe_oper = Mock()
 
-    with patch("app.chain.subscribe.SubscribeOper", return_value=subscribe_oper):
+    with patch("app.chain._music.SubscribeOper", return_value=subscribe_oper):
         SubscribeChain._sync_music_subscribe_target(subscribe, _music_info())
 
     subscribe_oper.update.assert_called_once_with(subscribe.id, {"total_tracks": None})