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/database/models.py

@@ -49,7 +49,7 @@ class TelegramBot(SQLModel, table=True):
     bot_username: str = Field(default="")
     bot_id: int = Field(default=0)
     webhook_path_id: str = Field(default_factory=lambda: uuid4().hex)
-    update_mode: str = Field(default="polling")  # "polling" or "webhook"
+    update_mode: str = Field(default="none")  # "none", "polling", or "webhook"
     # NOTE: commands_config column remains in the DB for backward compat,
     # but is no longer part of the SQLModel class. Data migrated to CommandConfig.
     created_at: datetime = Field(default_factory=_utcnow)