feat(tg-bot): unpin auto-forwarded channel posts

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.

platform/telegram/README.md

@@ -70,7 +70,12 @@ Telegram identity to an account from a browser. Both map a rejection to gRPC
   the bot applies it — but only to a member currently in the chat (it probes one user with
   `getChatMember`, since bots cannot list members). The bot must be an **administrator** there
   with the **"Ban users"** right (the Bot API `can_restrict_members`), and it subscribes to
-  `chat_member` updates, which Telegram delivers only to a chat admin.
+  `chat_member` updates, which Telegram delivers only to a chat admin. The bot also **removes
+  the automatic pin** Telegram places on every channel post auto-forwarded into this linked
+  chat (the `Message.is_automatic_forward` marker): it unpins only that one message
+  (`unpinChatMessage` by id), so a pin set by a human admin — or by the bot for another
+  message — is never touched. This needs the **pin-messages** right (`can_pin_messages`) in
+  addition to "Ban users"; a startup self-check warns when it is missing.
 - **Support relay (optional).** When `TELEGRAM_SUPPORT_CHAT_ID` names a private **forum
   supergroup**, the bot runs a direct support channel for users who message it. A user's first
   non-`/start` message opens a dedicated **forum topic** whose first message is an info card (the