feat(payments): payment-event dispatcher + the provider-return UX

Deliver payment_events to connected clients as an in-app wallet-refresh push: a
background dispatcher drains undispatched events and publishes a KindNotification
"payment" signal, marking each delivered; the client bumps a wallet-refresh
counter the open Wallet screen watches, re-fetching in place. A return-focus
refetch is the fallback. The Robokassa Success/Fail return now serves a
self-closing page (the payment opens in a separate window) so the customer drops
back into the live app instead of a cold start. Integration test for the event
drain/mark queue.

backend/internal/payments/store_intake.go

@@ -292,6 +292,42 @@ func (s *Store) insertPaymentEvent(ctx context.Context, accountID uuid.UUID, ord
 	return nil
 }
 
+// PaymentEvent is an undispatched lifecycle event the dispatcher delivers to an account.
+type PaymentEvent struct {
+	EventID   uuid.UUID
+	AccountID uuid.UUID
+	Type      string
+}
+
+// undispatchedEvents reads up to limit payment events not yet delivered, oldest first.
+func (s *Store) undispatchedEvents(ctx context.Context, limit int) ([]PaymentEvent, error) {
+	rows, err := s.db.QueryContext(ctx,
+		`SELECT event_id, account_id, type FROM payments.payment_events
+		 WHERE dispatched_at IS NULL ORDER BY created_at LIMIT $1`, limit)
+	if err != nil {
+		return nil, fmt.Errorf("payments: read undispatched events: %w", err)
+	}
+	defer rows.Close()
+	var out []PaymentEvent
+	for rows.Next() {
+		var e PaymentEvent
+		if err := rows.Scan(&e.EventID, &e.AccountID, &e.Type); err != nil {
+			return nil, fmt.Errorf("payments: scan event: %w", err)
+		}
+		out = append(out, e)
+	}
+	return out, rows.Err()
+}
+
+// markEventDispatched stamps an event as delivered so it is not re-sent.
+func (s *Store) markEventDispatched(ctx context.Context, eventID uuid.UUID, now time.Time) error {
+	if _, err := s.db.ExecContext(ctx,
+		`UPDATE payments.payment_events SET dispatched_at = $2 WHERE event_id = $1`, eventID, now); err != nil {
+		return fmt.Errorf("payments: mark event dispatched: %w", 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