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/internal/transcode/transcode.go

@@ -112,7 +112,7 @@ func NewRegistry(backend *backendclient.Client, tg TelegramValidator, opts ...Op
 	r.ops[MsgWalletGet] = Op{Handler: walletHandler(backend), Auth: true}
 	r.ops[MsgWalletCatalog] = Op{Handler: walletCatalogHandler(backend), Auth: true}
 	r.ops[MsgWalletBuy] = Op{Handler: walletBuyHandler(backend), Auth: true}
-	r.ops[MsgWalletOrder] = Op{Handler: walletOrderHandler(backend), Auth: true}
+	r.ops[MsgWalletOrder] = Op{Handler: walletOrderHandler(backend, nil), Auth: true}
 	r.ops[MsgBlockStatus] = Op{Handler: blockStatusHandler(backend), Auth: true}
 	r.ops[MsgGameSubmitPlay] = Op{Handler: submitPlayHandler(backend), Auth: true}
 	r.ops[MsgGameState] = Op{Handler: gameStateHandler(backend), Auth: true}
@@ -170,6 +170,17 @@ func WithVKLink(ex VKIDExchanger) Option {
 	}
 }
 
+// WithTelegramStars re-registers the wallet.order op with a Telegram Stars invoice minter (the
+// bot-link), so a Telegram-context order mints its Stars invoice link through the bot. A nil minter
+// is a no-op, leaving the default (Stars-unavailable) handler in place.
+func WithTelegramStars(minter InvoiceMinter) Option {
+	return func(r *Registry, backend *backendclient.Client) {
+		if minter != nil {
+			r.ops[MsgWalletOrder] = Op{Handler: walletOrderHandler(backend, minter), Auth: true}
+		}
+	}
+}
+
 // Lookup returns the operation for messageType, and whether it is registered.
 func (r *Registry) Lookup(messageType string) (Op, bool) {
 	op, ok := r.ops[messageType]
@@ -333,13 +344,37 @@ func walletBuyHandler(backend *backendclient.Client) Handler {
 	}
 }
 
-func walletOrderHandler(backend *backendclient.Client) Handler {
+// InvoiceMinter mints a Telegram Stars invoice link for a created order via the bot (only the bot
+// reaches Telegram). The gateway bot-link Hub implements it; it is nil where the bot-link is off,
+// which leaves the Telegram rail unavailable.
+type InvoiceMinter interface {
+	MintInvoice(ctx context.Context, title, description, orderID string, amountStars int64) (string, error)
+}
+
+// errTelegramStarsUnavailable is returned when a Telegram Stars order is requested but no invoice
+// minter (the bot-link) is wired — the rail is not available on this gateway.
+var errTelegramStarsUnavailable = errors.New("telegram stars rail unavailable")
+
+func walletOrderHandler(backend *backendclient.Client, minter InvoiceMinter) Handler {
 	return func(ctx context.Context, req Request) ([]byte, error) {
 		in := fb.GetRootAsWalletOrderRequest(req.Payload, 0)
 		o, err := backend.WalletOrder(ctx, req.UserID, string(in.ProductId()))
 		if err != nil {
 			return nil, err
 		}
+		// Telegram Stars: mint the invoice link here via the bot-link and return it to the client
+		// as the redirect URL it opens with WebApp.openInvoice. The title doubles as the invoice
+		// description (both are required and the pack title is the whole product).
+		if o.Rail == "telegram" {
+			if minter == nil {
+				return nil, errTelegramStarsUnavailable
+			}
+			link, merr := minter.MintInvoice(ctx, o.InvoiceTitle, o.InvoiceTitle, o.OrderID, o.InvoiceAmount)
+			if merr != nil {
+				return nil, merr
+			}
+			o.RedirectURL = link
+		}
 		return encodeWalletOrder(o), nil
 	}
 }