feat(payments): Telegram Stars payment rail

Accept real money via Telegram Stars (XTR) — the third intake rail
alongside Robokassa (direct) and VK Votes.

Only the bot reaches Telegram, so the rail funnels through the reverse
mTLS bot-link:
- the gateway mints the invoice on a CreateInvoice command (the bot
  calls createInvoiceLink, XTR; the link goes to WebApp.openInvoice);
- the bot gates each pre_checkout_query via a ValidatePreCheckout unary
  (the order must exist, be still creditable and not already paid — the
  reusable-invoice double-pay guard; the decline reason is localised to
  the order account's language);
- a completed successful_payment is queued in a durable pure-Go SQLite
  outbox and forwarded via a ForwardPayment unary, credited once
  (idempotent on telegram_payment_charge_id, honours an expired order),
  re-driven on restart and every 30s.

The rail is wired by TELEGRAM_STARS_OUTBOX_DIR (default /data) but stays
inert until a chip pack carries an XTR price, so seeding a Stars price in
the admin is the go-live.

Tests: backend integration (order->forward->credit once, duplicate,
pre_checkout gate) + bot outbox unit (idempotent, restart re-drive) +
executor createInvoice. Docs: PAYMENTS(+ru) §9, ARCHITECTURE, the
platform/telegram README, PLAN.

gateway/cmd/gateway/main.go

@@ -153,6 +153,16 @@ func run(ctx context.Context, cfg config.Config, logger *zap.Logger) error {
 				r, rerr := backend.ChatEligibility(ctx, externalID)
 				return r.Registered, r.Eligible, rerr
 			})
+		// The Telegram Stars payment bridge rides the same bot-link: the bot validates each
+		// pre_checkout and forwards each completed payment through these, backed by the backend intake.
+		botHub.SetPaymentBridge(
+			func(ctx context.Context, orderID string, amount int64, currency string) (bool, string, error) {
+				return backend.ValidatePreCheckout(ctx, orderID, amount, currency)
+			},
+			func(ctx context.Context, orderID, chargeID string, amount, telegramUserID int64) (bool, error) {
+				return backend.TelegramPayment(ctx, orderID, chargeID, amount, telegramUserID)
+			},
+		)
 		tlsCfg, terr := mtls.ServerConfig(cfg.BotLink.CertFile, cfg.BotLink.KeyFile, cfg.BotLink.CAFile)
 		if terr != nil {
 			return terr
@@ -199,7 +209,12 @@ func run(ctx context.Context, cfg config.Config, logger *zap.Logger) error {
 		vkidExchanger = vkid.New(cfg.VKID.AppID, cfg.VKID.ClientSecret, cfg.VKID.RedirectURI)
 		logger.Info("vk id web login enabled")
 	}
-	registry := transcode.NewRegistry(backend, validator, transcode.WithVKAuth(cfg.VKAppSecret), transcode.WithVKLink(vkidExchanger))
+	regOpts := []transcode.Option{transcode.WithVKAuth(cfg.VKAppSecret), transcode.WithVKLink(vkidExchanger)}
+	if botHub != nil {
+		// A Telegram-context wallet order mints its Stars invoice link through the connected bot.
+		regOpts = append(regOpts, transcode.WithTelegramStars(botHub))
+	}
+	registry := transcode.NewRegistry(backend, validator, regOpts...)
 	edge := connectsrv.NewServer(connectsrv.Deps{
 		Registry:     registry,
 		Sessions:     sessions,