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/backendclient/api.go

@@ -401,19 +401,60 @@ type walletOrderBody struct {
 	ProductID string `json:"product_id"`
 }
 
-// WalletOrderResp is a created order: its id and the provider launch URL the client opens.
+// WalletOrderResp is a created order: its id and the rail's launch details. RedirectURL is the
+// provider's hosted-payment URL (direct); Rail names the settling rail; for the Telegram Stars rail
+// InvoiceTitle and InvoiceAmount (whole stars) are the invoice the gateway mints via the bot.
 type WalletOrderResp struct {
-	OrderID     string `json:"order_id"`
-	RedirectURL string `json:"redirect_url"`
+	OrderID       string `json:"order_id"`
+	RedirectURL   string `json:"redirect_url"`
+	Rail          string `json:"rail"`
+	InvoiceTitle  string `json:"invoice_title"`
+	InvoiceAmount int64  `json:"invoice_amount"`
 }
 
-// WalletOrder opens a pending order to fund a chip pack and returns the provider launch URL.
+// WalletOrder opens a pending order to fund a chip pack and returns the rail's launch details.
 func (c *Client) WalletOrder(ctx context.Context, userID, productID string) (WalletOrderResp, error) {
 	var out WalletOrderResp
 	err := c.do(ctx, http.MethodPost, "/api/v1/user/wallet/order", userID, "", walletOrderBody{ProductID: productID}, &out)
 	return out, err
 }
 
+// preCheckoutResp is the backend intake's pre_checkout answer.
+type preCheckoutResp struct {
+	OK     bool   `json:"ok"`
+	Reason string `json:"reason"`
+}
+
+// ValidatePreCheckout asks the backend intake whether a Telegram Stars pre_checkout_query for
+// orderID paying amount in currency may be approved, before the charge. It returns the approval and
+// a short decline reason for the payer. It backs the bot's pre_checkout gate through the bot-link.
+func (c *Client) ValidatePreCheckout(ctx context.Context, orderID string, amount int64, currency string) (bool, string, error) {
+	var out preCheckoutResp
+	err := c.do(ctx, http.MethodPost, "/api/v1/internal/payments/telegram/precheckout", "", "",
+		map[string]any{"order_id": orderID, "amount": amount, "currency": currency}, &out)
+	return out.OK, out.Reason, err
+}
+
+// telegramPaymentResp is the backend intake's credit outcome.
+type telegramPaymentResp struct {
+	Credited bool `json:"credited"`
+}
+
+// TelegramPayment forwards a completed Telegram Stars payment from the bot's outbox to the backend
+// intake, which credits the order idempotently on chargeID. It reports whether the order was
+// credited (or already had been); a transport error is a transient failure the bot retries.
+func (c *Client) TelegramPayment(ctx context.Context, orderID, chargeID string, amount, telegramUserID int64) (bool, error) {
+	var out telegramPaymentResp
+	err := c.do(ctx, http.MethodPost, "/api/v1/internal/payments/telegram/payment", "", "",
+		map[string]any{
+			"order_id":                   orderID,
+			"telegram_payment_charge_id": chargeID,
+			"amount":                     amount,
+			"telegram_user_id":           telegramUserID,
+		}, &out)
+	return out.Credited, err
+}
+
 // robokassaResultResp is the backend intake's reply: the body to echo back to Robokassa.
 type robokassaResultResp struct {
 	Response string `json:"response"`