feat(payments): direct-rail order + intake handlers, config and reaper

Wire the Robokassa direct rail into the backend transport. POST
/api/v1/user/wallet/order (walletGate + a D36 confirmed-email gate for the
direct rail) opens a pending order and returns the signed Robokassa payment
URL. The internal, gateway-only /payments/robokassa/result endpoint verifies
the Result signature, credits the matched order exactly once via Fund (honoured
even if expired), records a succeeded payment event, and answers Robokassa's
"OK<InvId>". Add the Robokassa env config, an account HasConfirmedEmail check
(D36), the payment_events writer, and a periodic pending-order reaper. The
routes register only when a Robokassa merchant login is configured.

backend/internal/payments/store_intake.go

@@ -271,6 +271,27 @@ func (s *Store) fund(ctx context.Context, orderID uuid.UUID, provider, providerP
 	return outcome, nil
 }
 
+// insertPaymentEvent appends an undispatched lifecycle event (succeeded/failed/refunded) for the
+// dispatcher to deliver. orderID and payload (a jsonb detail blob) are optional.
+func (s *Store) insertPaymentEvent(ctx context.Context, accountID uuid.UUID, orderID *uuid.UUID, eventType string, payload []byte, now time.Time) error {
+	id, err := uuid.NewV7()
+	if err != nil {
+		return fmt.Errorf("payments: event id: %w", err)
+	}
+	var pl any = postgres.NULL
+	if payload != nil {
+		pl = string(payload)
+	}
+	stmt := table.PaymentEvents.INSERT(
+		table.PaymentEvents.EventID, table.PaymentEvents.AccountID, table.PaymentEvents.OrderID,
+		table.PaymentEvents.Type, table.PaymentEvents.Payload, table.PaymentEvents.CreatedAt,
+	).VALUES(id, accountID, uuidOrNull(orderID), eventType, pl, now)
+	if _, err := stmt.ExecContext(ctx, s.db); err != nil {
+		return fmt.Errorf("payments: insert %s event: %w", eventType, err)
+	}
+	return nil
+}
+
 // orderTTL reads the configured pending-order lifetime in whole seconds.
 func (s *Store) orderTTL(ctx context.Context) (int, error) {
 	var cfg model.Config