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.

frontend/src/lib/i18n/en.json

@@ -433,6 +433,8 @@
 		"webhookRegistered": "Webhook registered",
 		"webhookUnregistered": "Webhook unregistered",
 		"updateMode": "Update mode",
+		"none": "None",
+		"noneActive": "Listener disabled",
 		"polling": "Polling",
 		"webhook": "Webhook",
 		"webhookStatus": "Webhook status",

frontend/src/lib/i18n/ru.json

@@ -433,6 +433,8 @@
 		"webhookRegistered": "Вебхук зарегистрирован",
 		"webhookUnregistered": "Вебхук удалён",
 		"updateMode": "Режим обновлений",
+		"none": "Откл.",
+		"noneActive": "Приём обновлений отключён",
 		"polling": "Опрос",
 		"webhook": "Вебхук",
 		"webhookStatus": "Статус вебхука",