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/migrations.py

@@ -144,7 +144,10 @@ async def migrate_schema(engine: AsyncEngine) -> None:
                 if bots:
                     logger.info("Backfilled webhook_path_id for %d existing bots", len(bots))
 
-            # Add update_mode to telegram_bot if missing
+            # Add update_mode to telegram_bot if missing.
+            # Existing bots pre-date this feature and were implicitly polling;
+            # preserve that behavior. New bots default to "none" via the
+            # SQLModel field default on fresh schemas.
             if not await _has_column(conn, "telegram_bot", "update_mode"):
                 await conn.execute(
                     text("ALTER TABLE telegram_bot ADD COLUMN update_mode TEXT DEFAULT 'polling'")