Telegram non-disableably auto-pins each channel post it auto-forwards
into the linked discussion group. The bot now detects that message by
Message.is_automatic_forward in the moderated chat (TELEGRAM_CHAT_ID)
and unpins it by id, so a pin set by a human admin — or by the bot for
another message — is never touched (no unpinAllChatMessages).
Needs the can_pin_messages right in the chat; the startup self-check
now also warns when it is missing. Bot-only; no wire/schema/DB change.
Ilia Denisov