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/botlink/command.go

@@ -48,3 +48,18 @@ func ChatGateCommand(externalID string, allow bool) *botlinkv1.Command {
 		}},
 	}
 }
+
+// CreateInvoiceCommand builds a Telegram Stars invoice-mint command: the bot calls
+// createInvoiceLink (in XTR) with the given title and description, the order id as the
+// payload (echoed by Telegram in pre_checkout and successful_payment) and amountStars whole
+// stars, and returns the link in its Ack result.
+func CreateInvoiceCommand(title, description, orderID string, amountStars int64) *botlinkv1.Command {
+	return &botlinkv1.Command{
+		Payload: &botlinkv1.Command_CreateInvoice{CreateInvoice: &botlinkv1.CreateInvoiceCommand{
+			Title:       title,
+			Description: description,
+			Payload:     orderID,
+			Amount:      amountStars,
+		}},
+	}
+}