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.

gateway/internal/botlink/command.go

@@ -36,3 +36,15 @@ func SendToGameChannelCommand(text string) *botlinkv1.Command {
 		}},
 	}
 }
+
+// ChatGateCommand builds a chat-gate command that sets whether the Telegram user
+// identified by externalID may write in the moderated discussion chat. The bot
+// applies it only to a member currently in the chat (guarded on getChatMember).
+func ChatGateCommand(externalID string, allow bool) *botlinkv1.Command {
+	return &botlinkv1.Command{
+		Payload: &botlinkv1.Command_ChatGate{ChatGate: &botlinkv1.ChatGateCommand{
+			ExternalId: externalID,
+			Allow:      allow,
+		}},
+	}
+}