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/cmd/backend/main.go

@@ -308,6 +308,7 @@ func run(ctx context.Context, cfg config.Config, logger *zap.Logger) error {
 		Notifier:      hub,
 		ExportSignKey: cfg.ExportSignKey,
 		Renderer:      renderer,
+		Robokassa:     cfg.Robokassa,
 	})
 	pushSrv := pushgrpc.NewServer(cfg.GRPCAddr, hub, logger)
 
@@ -316,6 +317,10 @@ func run(ctx context.Context, cfg config.Config, logger *zap.Logger) error {
 	logger.Info("servers starting",
 		zap.String("http_addr", cfg.HTTPAddr),
 		zap.String("grpc_addr", cfg.GRPCAddr))
+	// Sweep expired pending payment orders on a cadence (cosmetic hygiene; a late valid callback
+	// still credits). Runs until ctx is cancelled.
+	go runOrderReaper(ctx, paymentsSvc, logger)
+
 	errc := make(chan error, 2)
 	go func() { errc <- pushSrv.Run(ctx) }()
 	go func() { errc <- srv.Run(ctx) }()
@@ -325,6 +330,26 @@ func run(ctx context.Context, cfg config.Config, logger *zap.Logger) error {
 	return err
 }
 
+// runOrderReaper periodically expires pending payment orders past their configured lifetime, until
+// ctx is cancelled. Expiry is cosmetic: a later valid provider callback still credits an expired
+// order.
+func runOrderReaper(ctx context.Context, p *payments.Service, log *zap.Logger) {
+	t := time.NewTicker(5 * time.Minute)
+	defer t.Stop()
+	for {
+		select {
+		case <-ctx.Done():
+			return
+		case <-t.C:
+			if n, err := p.ExpireOrders(ctx); err != nil {
+				log.Warn("order reaper: sweep failed", zap.Error(err))
+			} else if n > 0 {
+				log.Info("order reaper: expired pending orders", zap.Int("count", n))
+			}
+		}
+	}
+}
+
 // newMailer builds the confirm-code mailer: an SMTP relay when a host is
 // configured, otherwise the development log mailer (the code is logged, not sent).
 func newMailer(cfg account.SMTPConfig, logger *zap.Logger) account.Mailer {