refactor(chain): chain 与 module 仅经 run_module 契约互联，实现模块内容封闭

- mTorrent 字幕链接解析下沉为 IndexerModule.site_subtitle_links，subtitle 模块自行跳过 API 站点
- TMDB/MusicBrainz 识别缓存管理改为模块方法（tmdb_cache_*/music_cache_*）经 run_module 分发
- WechatClawBot 客户端查找/临时客户端/缓存迁移内聚为 wechatclawbot_* 模块方法，
  chain 移除 WechatClawBot 类与 ModuleManager 内省
- LISTENBRAINZ_* 常量迁至 schemas/types.py，模块与链层再导入保持兼容
- TMDbException 升为 schemas/exception.py 跨层契约，vendored 异常保持类身份一致
- 架构守护测试新增 test_chain_does_not_import_module_internals（共 18 项）
- 文档同步：chain->module 仅允许 run_module 分发，直接导入禁止

app/api/endpoints/notification.py

@@ -27,7 +27,7 @@ def wechatclawbot_status(
     _: User = Depends(get_current_active_superuser),
 ):
     """查询微信 ClawBot 登录状态和二维码。"""
-    client, errmsg = MessageChain.get_wechatclawbot_client(
+    client, errmsg = MessageChain().get_wechatclawbot_client(
         source=source,
         fallback_source=fallback_source,
         WECHATCLAWBOT_BASE_URL=WECHATCLAWBOT_BASE_URL,
@@ -62,7 +62,7 @@ def refresh_wechatclawbot_qrcode(
     _: User = Depends(get_current_active_superuser),
 ):
     """刷新微信 ClawBot 二维码。"""
-    client, errmsg = MessageChain.get_wechatclawbot_client(
+    client, errmsg = MessageChain().get_wechatclawbot_client(
         source=source,
         fallback_source=fallback_source,
         WECHATCLAWBOT_BASE_URL=WECHATCLAWBOT_BASE_URL,
@@ -96,7 +96,7 @@ def logout_wechatclawbot(
     _: User = Depends(get_current_active_superuser),
 ):
     """退出微信 ClawBot 登录。"""
-    client, errmsg = MessageChain.get_wechatclawbot_client(
+    client, errmsg = MessageChain().get_wechatclawbot_client(
         source=source,
         fallback_source=fallback_source,
         WECHATCLAWBOT_BASE_URL=WECHATCLAWBOT_BASE_URL,
@@ -130,7 +130,7 @@ def test_wechatclawbot(
     _: User = Depends(get_current_active_superuser),
 ):
     """测试微信 ClawBot 当前登录态是否可用。"""
-    client, errmsg = MessageChain.get_wechatclawbot_client(
+    client, errmsg = MessageChain().get_wechatclawbot_client(
         source=source,
         fallback_source=fallback_source,
         WECHATCLAWBOT_BASE_URL=WECHATCLAWBOT_BASE_URL,
@@ -158,7 +158,7 @@ def migrate_wechatclawbot_cache(
     _: User = Depends(get_current_active_superuser),
 ):
     """在通知名称变更时迁移对应的微信 ClawBot 登录缓存。"""
-    success, message = MessageChain.migrate_wechatclawbot_cache(
+    success, message = MessageChain().migrate_wechatclawbot_cache(
         old_name=old_source,
         new_name=new_source,
         cleanup_old=cleanup_old,