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/inttest/payments_intake_test.go

@@ -129,6 +129,101 @@ func TestPaymentsVKOrderFundCredits(t *testing.T) {
 	}
 }
 
+// TestPaymentsTelegramStarsRail exercises the Telegram Stars rail over Postgres: an order prices the
+// pack in whole stars (XTR); a pre_checkout on the pending order is approved; the forwarded payment
+// credits the telegram segment exactly once (idempotent on the Telegram charge id); and a
+// pre_checkout after the order is paid is declined (a reusable invoice link paid twice).
+func TestPaymentsTelegramStarsRail(t *testing.T) {
+	ctx := context.Background()
+	svc := newPaymentsService()
+	acc := uuid.New()
+	prod := seedPackProduct(t, 50, methodPrice{method: "telegram", currency: "XTR", amount: 40}) // 40 stars fund 50 chips
+
+	res, err := svc.CreateOrder(ctx, acc, payments.NewContext("telegram", "android"), []payments.Source{payments.SourceTelegram}, prod, "telegram")
+	if err != nil {
+		t.Fatalf("create telegram order: %v", err)
+	}
+	if res.Amount.Currency() != payments.CurrencyStar || res.Amount.Minor() != 40 {
+		t.Fatalf("telegram order amount = %s, want 40 XTR", res.Amount)
+	}
+
+	// pre_checkout on the pending order: approved, and it reports the account for reason localisation.
+	starAmt, _ := payments.MoneyFromMinor(40, payments.CurrencyStar)
+	pc, err := svc.ValidatePreCheckout(ctx, res.OrderID, starAmt)
+	if err != nil {
+		t.Fatalf("pre_checkout: %v", err)
+	}
+	if !pc.OK || pc.AccountID != acc {
+		t.Fatalf("pre_checkout = %+v, want approved for account %s", pc, acc)
+	}
+
+	// The forwarded successful_payment credits once, idempotent on the Telegram charge id.
+	out, err := svc.Fund(ctx, res.OrderID, "telegram", "tg-charge-1", starAmt)
+	if err != nil {
+		t.Fatalf("telegram fund: %v", err)
+	}
+	if out.AlreadyCredited || out.Chips != 50 || out.Source != payments.SourceTelegram {
+		t.Fatalf("telegram fund outcome = %+v, want 50 chips credited to telegram", out)
+	}
+	if got := readBalance(t, acc, "telegram"); got != 50 {
+		t.Errorf("telegram balance after fund = %d, want 50", got)
+	}
+
+	// A retried forward (same charge id, e.g. a lost ack) credits nothing more.
+	out2, err := svc.Fund(ctx, res.OrderID, "telegram", "tg-charge-1", starAmt)
+	if err != nil {
+		t.Fatalf("duplicate telegram fund: %v", err)
+	}
+	if !out2.AlreadyCredited {
+		t.Error("retried forward not flagged AlreadyCredited")
+	}
+	if got := readBalance(t, acc, "telegram"); got != 50 {
+		t.Errorf("telegram balance after retry = %d, want 50 (credited once)", got)
+	}
+
+	// pre_checkout after the order is paid: declined (the reusable-link double-pay guard).
+	pc2, err := svc.ValidatePreCheckout(ctx, res.OrderID, starAmt)
+	if err != nil {
+		t.Fatalf("pre_checkout after paid: %v", err)
+	}
+	if pc2.OK || pc2.Reason != payments.PreCheckoutAlreadyPaid {
+		t.Errorf("pre_checkout after paid = %+v, want a decline with already_paid", pc2)
+	}
+}
+
+// TestPaymentsTelegramPreCheckoutDeclines covers the pre_checkout decline reasons: an unknown order
+// and an amount that no longer matches.
+func TestPaymentsTelegramPreCheckoutDeclines(t *testing.T) {
+	ctx := context.Background()
+	svc := newPaymentsService()
+	starAmt, _ := payments.MoneyFromMinor(40, payments.CurrencyStar)
+
+	// An unknown order id declines as gone, with no account to localise against.
+	pc, err := svc.ValidatePreCheckout(ctx, uuid.New(), starAmt)
+	if err != nil {
+		t.Fatalf("pre_checkout unknown: %v", err)
+	}
+	if pc.OK || pc.Reason != payments.PreCheckoutGone || pc.AccountID != (uuid.UUID{}) {
+		t.Errorf("pre_checkout unknown = %+v, want a gone decline with no account", pc)
+	}
+
+	// A pending order validated at the wrong amount declines as price-changed.
+	acc := uuid.New()
+	prod := seedPackProduct(t, 100, methodPrice{method: "telegram", currency: "XTR", amount: 80})
+	res, err := svc.CreateOrder(ctx, acc, payments.NewContext("telegram", "android"), []payments.Source{payments.SourceTelegram}, prod, "telegram")
+	if err != nil {
+		t.Fatalf("create telegram order: %v", err)
+	}
+	wrong, _ := payments.MoneyFromMinor(40, payments.CurrencyStar)
+	pc2, err := svc.ValidatePreCheckout(ctx, res.OrderID, wrong)
+	if err != nil {
+		t.Fatalf("pre_checkout mismatch: %v", err)
+	}
+	if pc2.OK || pc2.Reason != payments.PreCheckoutPriceChanged {
+		t.Errorf("pre_checkout mismatch = %+v, want a price_changed decline", pc2)
+	}
+}
+
 // TestPaymentsFundAmountMismatch verifies a callback whose paid amount does not match the order is
 // refused and credits nothing (§9: verify the amount after matching by order id).
 func TestPaymentsFundAmountMismatch(t *testing.T) {