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/events.go

@@ -216,6 +216,16 @@ func BannerChanged(userID uuid.UUID) Intent {
 	return Notification(userID, NotifyBanner)
 }
 
+// ChatAccessChanged signals that userID's eligibility to write in the moderated
+// Telegram discussion chat may have changed (an admin block/unblock, a chat_muted
+// grant/revoke, or a temporary block lapsing). It carries no payload: the gateway
+// resolves the user's Telegram identity and current eligibility and pushes the
+// resulting chat-gate command to the bot. Unlike the lobby notifications it is an
+// infra signal — a distinct top-level kind, never an out-of-app rendered message.
+func ChatAccessChanged(userID uuid.UUID) Intent {
+	return Intent{UserID: userID, Kind: KindChatAccessChanged, EventID: eventID()}
+}
+
 // eventID returns a best-effort correlation id for one emitted event.
 func eventID() string {
 	if id, err := uuid.NewV7(); err == nil {