refactor(schemas): 统一 message/notification 命名边界，旧名收敛至兼容映射表

- notification 域：渠道能力（MessageChannel→NotificationChannel、ChannelCapability* 迁入 notification.py）
- message 域：消息收发（Notification→Message、NotificationType→MessageType、CommingMessage→IncomingMessage、NotificationHistoryItem→MessageHistoryItem、NotificationClear*→MessageClear*）
- Agent 工具契约：send_notification_message→send_message、notification_callback→message_callback
- 源码不保留旧名物理别名，旧导入经 app/runtime/compat/manifest.py SYMBOL_ALIASES 惰性解析
- API 路径与持久化键冻结不变，前端零改动
- 新增兼容守护测试与 docs/rules/07 命名边界规范

tests/test_downloader_path_mapping.py

@@ -44,7 +44,7 @@ def _load_downloader_base():
     schema_types_module.ModuleType = Enum("ModuleType", {"Downloader": "downloader"})
     schema_types_module.DownloaderType = Enum("DownloaderType", {"Qbittorrent": "Qbittorrent"})
     schema_types_module.MediaServerType = Enum("MediaServerType", {"Emby": "Emby"})
-    schema_types_module.MessageChannel = Enum("MessageChannel", {"Telegram": "telegram"})
+    schema_types_module.NotificationChannel = Enum("NotificationChannel", {"Telegram": "telegram"})
     schema_types_module.OtherModulesType = Enum("OtherModulesType", {"Subtitle": "subtitle"})
     schema_types_module.MediaRecognizeType = Enum(
         "MediaRecognizeType", {"TheMovieDb": "themoviedb"}
@@ -60,7 +60,7 @@ def _load_downloader_base():
 
     service_module.ServiceConfigHelper = _ServiceConfigHelper
     mixins_module.ConfigReloadMixin = _ConfigReloadMixin
-    schemas_module.Notification = object
+    schemas_module.Message = object
     schemas_module.NotificationConf = object
     schemas_module.MediaServerConf = object
     schemas_module.DownloaderConf = object