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.
alexei.dolgolyov
be15463fd2