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/store_wallet.go

@@ -230,8 +230,11 @@ func applyBenefitTx(ctx context.Context, tx *sql.Tx, accountID uuid.UUID, origin
 	return nil
 }
 
-// insertLedgerTx appends one append-only ledger row inside tx.
-func insertLedgerTx(ctx context.Context, tx *sql.Tx, accountID uuid.UUID, kind string, source, origin *Source, chipsDelta int, productID *uuid.UUID, snapshot []byte, now time.Time) error {
+// insertLedgerTx appends one append-only ledger row inside tx. orderID, provider and
+// providerPaymentID are set only on an intake credit (fund/refund) and are nil for a
+// spend/admin_grant; a non-nil (provider, providerPaymentID) pair is guarded by the partial
+// unique index, so a duplicate provider callback fails here (the idempotency key).
+func insertLedgerTx(ctx context.Context, tx *sql.Tx, accountID uuid.UUID, kind string, source, origin *Source, chipsDelta int, productID, orderID *uuid.UUID, provider, providerPaymentID *string, snapshot []byte, now time.Time) error {
 	id, err := uuid.NewV7()
 	if err != nil {
 		return fmt.Errorf("payments: ledger id: %w", err)
@@ -246,11 +249,13 @@ func insertLedgerTx(ctx context.Context, tx *sql.Tx, accountID uuid.UUID, kind s
 	stmt := table.Ledger.INSERT(
 		table.Ledger.LedgerID, table.Ledger.AccountID, table.Ledger.Kind,
 		table.Ledger.Source, table.Ledger.Origin, table.Ledger.ChipsDelta,
-		table.Ledger.ProductID, table.Ledger.Snapshot, table.Ledger.CreatedAt,
+		table.Ledger.ProductID, table.Ledger.OrderID, table.Ledger.Provider,
+		table.Ledger.ProviderPaymentID, table.Ledger.Snapshot, table.Ledger.CreatedAt,
 	).VALUES(
 		id, accountID, kind,
 		sourceOrNull(source), sourceOrNull(origin), int32(chipsDelta),
-		uuidOrNull(productID), snap, now,
+		uuidOrNull(productID), uuidOrNull(orderID), stringOrNull(provider),
+		stringOrNull(providerPaymentID), snap, now,
 	)
 	if _, err := stmt.ExecContext(ctx, tx); err != nil {
 		return fmt.Errorf("payments: insert %s ledger: %w", kind, err)
@@ -278,7 +283,7 @@ func (s *Store) spend(ctx context.Context, accountID uuid.UUID, draws []sourceAm
 				return ErrInsufficientChips
 			}
 			src := dr.source
-			if err := insertLedgerTx(ctx, tx, accountID, "spend", &src, &origin, -dr.amount, &productID, snapshot, now); err != nil {
+			if err := insertLedgerTx(ctx, tx, accountID, "spend", &src, &origin, -dr.amount, &productID, nil, nil, nil, snapshot, now); err != nil {
 				return err
 			}
 		}
@@ -295,7 +300,7 @@ func (s *Store) spend(ctx context.Context, accountID uuid.UUID, draws []sourceAm
 // the chosen origin, in one transaction — a zero-price sale of a value.
 func (s *Store) grant(ctx context.Context, accountID uuid.UUID, origin Source, d benefitDelta, snapshot []byte, now time.Time) error {
 	err := withTx(ctx, s.db, func(tx *sql.Tx) error {
-		if err := insertLedgerTx(ctx, tx, accountID, "admin_grant", nil, &origin, 0, nil, snapshot, now); err != nil {
+		if err := insertLedgerTx(ctx, tx, accountID, "admin_grant", nil, &origin, 0, nil, nil, nil, nil, snapshot, now); err != nil {
 			return err
 		}
 		return applyBenefitTx(ctx, tx, accountID, origin, d, now)
@@ -419,3 +424,11 @@ func uuidOrNull(id *uuid.UUID) postgres.Expression {
 	}
 	return postgres.UUID(*id)
 }
+
+// stringOrNull renders an optional string as a SQL string or NULL.
+func stringOrNull(s *string) postgres.Expression {
+	if s == nil {
+		return postgres.NULL
+	}
+	return postgres.String(*s)
+}