feat(telegram): promo bot + channel-chat moderation gate

Add a second standalone promo bot to the bot container (answers /start with a
localized message + a URL button into the main bot's Mini App) and gate write
access in a channel's linked discussion chat: grant on join when the Telegram
user is registered and neither admin-suspended nor holding a new chat_muted
role, and revoke/grant on the matching moderation change for a member currently
in the chat.

Eligibility (registered AND NOT suspended AND NOT chat_muted; the game
suspension dominates) is resolved once in the backend and reached two ways: the
bot's join-time unary ResolveChatEligibility over the existing mTLS bot-link,
and a backend chat_access_changed event -> gateway -> ChatGate command
(idempotent; a temporary-block-expiry sweeper may over-emit). The bot guards the
block/unblock path with getChatMember, since bots cannot list members.

A web_app button cannot open another bot's Mini App (it signs initData with the
sending bot's token), so the promo button is a t.me ?startapp URL reusing the
UI's VITE_TELEGRAM_LINK. The bot must be a chat admin with the restrict-members
right and chat_member in its allowed updates.

No schema change: chat_muted reuses the data-driven account_roles table.

backend/internal/notify/notify.go

@@ -35,6 +35,13 @@ const (
 	// KindGameOver announces a finished game to each seated player, driving the
 	// out-of-app "game over" push.
 	KindGameOver = "game_over"
+	// KindChatAccessChanged signals that a player's eligibility to write in the
+	// moderated Telegram discussion chat may have changed (an admin block or unblock,
+	// a chat_muted grant or revoke, or a temporary block lapsing). It carries no
+	// payload and is never fanned out to in-app clients: the gateway consumes it to
+	// resolve the player's Telegram identity and current eligibility and push the
+	// resulting chat-gate command to the bot.
+	KindChatAccessChanged = "chat_access_changed"
 )
 
 // Notification sub-kinds carried in a KindNotification event payload; the client