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/payments/service_intake.go

@@ -2,6 +2,7 @@ package payments
 
 import (
 	"context"
+	"errors"
 	"fmt"
 
 	"github.com/google/uuid"
@@ -80,6 +81,49 @@ func (s *Service) Fund(ctx context.Context, orderID uuid.UUID, provider, provide
 	return s.store.fund(ctx, orderID, provider, providerPaymentID, paid, s.clock())
 }
 
+// Pre-checkout decline reason codes. They are language-neutral: the transport layer localises them
+// to the order account's preferred language before showing the payer (the reason is displayed in the
+// Telegram payment sheet).
+const (
+	// PreCheckoutGone means no order matches — an unknown or stale invoice payload.
+	PreCheckoutGone = "order_gone"
+	// PreCheckoutAlreadyPaid means the order is already paid (a reusable invoice link paid twice).
+	PreCheckoutAlreadyPaid = "already_paid"
+	// PreCheckoutPriceChanged means the amount or currency no longer matches the order.
+	PreCheckoutPriceChanged = "price_changed"
+)
+
+// PreCheckoutOutcome is the pre-charge validation of a Telegram Stars order. OK approves the charge;
+// otherwise Reason is a decline reason code the transport localises. AccountID is the order's account
+// (for localising the reason to its preferred language); it is the zero UUID when the order is unknown.
+type PreCheckoutOutcome struct {
+	OK        bool
+	Reason    string
+	AccountID uuid.UUID
+}
+
+// ValidatePreCheckout answers whether a Stars pre_checkout_query for orderID paying amount may be
+// approved, before any star is charged. It approves an order that exists, is not already paid (a
+// reusable invoice link paid a second time is refused here) and whose expected amount and currency
+// match the invoice. A pending or honoured-expired order is approved — a late credit is honoured
+// (§9/D23). A missing order or a mismatch is a clean decline with a reason code, not an error.
+func (s *Service) ValidatePreCheckout(ctx context.Context, orderID uuid.UUID, amount Money) (PreCheckoutOutcome, error) {
+	ord, err := s.store.orderByID(ctx, orderID)
+	if errors.Is(err, ErrOrderNotFound) {
+		return PreCheckoutOutcome{OK: false, Reason: PreCheckoutGone}, nil
+	}
+	if err != nil {
+		return PreCheckoutOutcome{}, err
+	}
+	if ord.status == "paid" {
+		return PreCheckoutOutcome{OK: false, Reason: PreCheckoutAlreadyPaid, AccountID: ord.accountID}, nil
+	}
+	if amount.Currency() != Currency(ord.currency) || amount.Minor() != ord.expectedAmount {
+		return PreCheckoutOutcome{OK: false, Reason: PreCheckoutPriceChanged, AccountID: ord.accountID}, nil
+	}
+	return PreCheckoutOutcome{OK: true, AccountID: ord.accountID}, nil
+}
+
 // ExpireOrders marks pending orders older than the configured lifetime as expired, returning how
 // many were swept. It backs the periodic pending reaper; expiry is cosmetic (a late valid callback
 // still credits — see Fund).