feat(telegram): add 'none' listener mode for bots

Introduce a third update_mode option alongside polling/webhook. 'none'
disables both polling and webhook delivery — useful when another instance
owns the listener or when the bot is send-only. Switching into 'none' now
unschedules polling and unregisters any active webhook so Telegram stops
delivering updates.

New bots default to 'none' (safer when multiple bridges share a token).
Existing bots upgraded from a pre-update_mode schema keep 'polling' so
their behavior is unchanged.

packages/server/src/notify_bridge_server/services/backup_schema.py

@@ -173,7 +173,7 @@ class TelegramBotData(BaseModel):
     token: str = ""
     icon: str = ""
     bot_username: str = ""
-    update_mode: str = "polling"
+    update_mode: str = "none"
 
 
 class MatrixBotData(BaseModel):