feat(payments): order-flow and fund credit engine + the Robokassa adapter

Add the payment-intake write path (provider-agnostic) and the Robokassa
direct-rail glue, both unit-tested; transport, wire and UI follow.

- payments: extend the ledger insert to thread order_id/provider/
  provider_payment_id (spend/grant pass nil); add the order store
  (create/read/expire + a pack-price loader) and the fund credit — a
  fund ledger row + a guarded balance upsert + mark-paid in one tx,
  idempotent on the (provider, provider_payment_id) unique index, cache
  invalidated after commit. A valid callback is honoured even on an
  expired order. Service CreateOrder/Fund/ExpireOrders; Money.Major for
  the provider amount field.
- robokassa: build the signed hosted-payment URL (SHA-256, order id via
  Shp_order, InvId unused) and verify the Result callback signature
  (Password2), extracting the order and amount. Receipt/fiscalisation is
  configured shop-side, so no Receipt parameter is sent.

backend/internal/payments/service_intake.go

@@ -0,0 +1,73 @@
+package payments
+
+import (
+	"context"
+	"fmt"
+
+	"github.com/google/uuid"
+)
+
+// OrderResult is what CreateOrder returns to the transport: the created order id and the details a
+// provider launch payload needs — the amount to charge and a human title for the payment.
+type OrderResult struct {
+	OrderID uuid.UUID
+	Amount  Money
+	Title   string
+}
+
+// CreateOrder opens a pending order to fund a chip pack in the execution context's payment method,
+// tagged with the provider that will settle it. It gate-checks the context (trusted, not the
+// VK-iOS spend freeze) and that the method's funding segment is attached, prices the pack in the
+// method's currency, then writes the order. The caller enforces any account-level precondition
+// (e.g. the direct email anchor, D36) before calling — payments holds no cross-schema identity
+// knowledge.
+func (s *Service) CreateOrder(ctx context.Context, accountID uuid.UUID, cxt Context, present []Source, productID uuid.UUID, provider string) (OrderResult, error) {
+	if !cxt.Trusted() || cxt.vkFrozen() {
+		return OrderResult{}, ErrUntrusted
+	}
+	method := cxt.Kind
+	if !has(present, method) {
+		return OrderResult{}, ErrUntrusted // the funding segment is not attached to the account
+	}
+	pack, err := s.store.loadPackForOrder(ctx, productID, method)
+	if err != nil {
+		return OrderResult{}, err
+	}
+	orderID, err := uuid.NewV7()
+	if err != nil {
+		return OrderResult{}, fmt.Errorf("payments: order id: %w", err)
+	}
+	o := newOrder{
+		orderID:   orderID,
+		accountID: accountID,
+		platform:  string(method),
+		productID: productID,
+		amount:    pack.price,
+		origin:    method,
+		provider:  provider,
+	}
+	if err := s.store.createOrder(ctx, o, s.clock()); err != nil {
+		return OrderResult{}, err
+	}
+	return OrderResult{OrderID: orderID, Amount: pack.price, Title: pack.title}, nil
+}
+
+// Fund credits a paid order into its funded segment exactly once, from a verified provider callback
+// — the single writer for every rail. It matches the order, verifies the paid amount, appends the
+// fund ledger row (idempotent on (provider, provider_payment_id)), credits the balance and marks
+// the order paid. A duplicate callback returns AlreadyCredited without a second credit; a valid
+// callback is honoured even on an expired order (§9/D23).
+func (s *Service) Fund(ctx context.Context, orderID uuid.UUID, provider, providerPaymentID string, paid Money) (FundOutcome, error) {
+	return s.store.fund(ctx, orderID, provider, providerPaymentID, paid, s.clock())
+}
+
+// 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).
+func (s *Service) ExpireOrders(ctx context.Context) (int, error) {
+	ttl, err := s.store.orderTTL(ctx)
+	if err != nil {
+		return 0, err
+	}
+	return s.store.expirePending(ctx, ttl, s.clock())
+}