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.

backend/internal/server/handlers.go

@@ -80,6 +80,10 @@ func (s *Server) registerRoutes() {
 		// the Robokassa Result callback when a merchant is configured.
 		u.POST("/wallet/order", s.handleWalletOrder)
 		s.internal.POST("/payments/vk/callback", s.handleVKCallback)
+		// The Telegram Stars rail: the bot forwards a pre_checkout validation and a completed
+		// payment over the reverse bot-link; the gateway proxies both onto these gateway-only routes.
+		s.internal.POST("/payments/telegram/precheckout", s.handleTelegramPreCheckout)
+		s.internal.POST("/payments/telegram/payment", s.handleTelegramPayment)
 		if s.robokassa.MerchantLogin != "" {
 			s.internal.POST("/payments/robokassa/result", s.handleRobokassaResult)
 		}