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/cmd/gateway/main.go

@@ -147,7 +147,11 @@ func run(ctx context.Context, cfg config.Config, logger *zap.Logger) error {
 	// fire-and-forget; the backend admin relay (plaintext, internal) awaits the Ack.
 	var botHub *botlink.Hub
 	if cfg.BotLinkEnabled() {
-		botHub = botlink.NewHub(logger, tel.MeterProvider().Meter("scrabble/gateway/botlink"))
+		botHub = botlink.NewHub(logger, tel.MeterProvider().Meter("scrabble/gateway/botlink"),
+			func(ctx context.Context, externalID string) (bool, bool, error) {
+				r, rerr := backend.ChatEligibility(ctx, externalID)
+				return r.Registered, r.Eligible, rerr
+			})
 		tlsCfg, terr := mtls.ServerConfig(cfg.BotLink.CertFile, cfg.BotLink.KeyFile, cfg.BotLink.CAFile)
 		if terr != nil {
 			return terr
@@ -361,6 +365,15 @@ func runPushPump(ctx context.Context, backend *backendclient.Client, hub *push.H
 				}
 				break
 			}
+			// A chat-access-changed event is an infra signal, not an in-app event:
+			// resolve the recipient's Telegram identity and current eligibility and
+			// push the chat-gate command to the bot, without fanning it out to clients.
+			if ev.GetKind() == chatAccessChangedKind {
+				if bot != nil {
+					go deliverChatGate(ctx, backend, bot, ev.GetUserId(), logger)
+				}
+				continue
+			}
 			hub.Publish(push.Event{
 				UserID:  ev.GetUserId(),
 				Kind:    ev.GetKind(),
@@ -398,6 +411,27 @@ func deliverOutOfApp(ctx context.Context, backend *backendclient.Client, bot *bo
 	bot.Send(botlink.NotifyCommand(target.ExternalID, kind, payload, target.Language))
 }
 
+// chatAccessChangedKind is the backend event signalling that a player's moderated-chat
+// write eligibility may have changed; the gateway turns it into a bot-link chat-gate
+// command rather than an in-app event (it mirrors notify.KindChatAccessChanged).
+const chatAccessChangedKind = "chat_access_changed"
+
+// deliverChatGate resolves a chat-access-changed event to the recipient's Telegram
+// identity and current eligibility and pushes the chat-gate command to the bot. It is
+// best-effort: a recipient with no Telegram identity is skipped, and a resolve failure
+// is logged and dropped (the next moderation action, or a re-join, re-applies the gate).
+func deliverChatGate(ctx context.Context, backend *backendclient.Client, bot *botlink.Hub, userID string, logger *zap.Logger) {
+	res, err := backend.ChatAccessByUser(ctx, userID)
+	if err != nil {
+		logger.Warn("chat-gate resolve failed", zap.String("user_id", userID), zap.Error(err))
+		return
+	}
+	if res.ExternalID == "" {
+		return // no Telegram identity, nothing to gate
+	}
+	bot.Send(botlink.ChatGateCommand(res.ExternalID, res.Eligible))
+}
+
 // sleep waits for d or until ctx is cancelled, reporting whether it waited the
 // full duration.
 func sleep(ctx context.Context, d time.Duration) bool {