feat(admin): manual full-order refund + ledger CSV export

Closes the admin / reports / catalog work. Each fund row on the /_gm finance
panel gains a Refund action (payments.RefundOrderFull): a full-order refund the
operator records after refunding on the rail — a refund ledger row + a floor-0
chip revoke (never negative, D27), idempotent (a second refund reports
already-refunded). A ledger CSV export (/_gm/ledger.csv, payments.LedgerExport)
streams the whole append-only ledger for tax + reconciliation.

Tests: refund an order in full (chips revoked, a refund row), an idempotent
second refund, the CSV export shape; CSRF-guarded.

backend/internal/payments/refund_admin.go

@@ -0,0 +1,41 @@
+package payments
+
+import (
+	"context"
+
+	"github.com/google/uuid"
+)
+
+// providerAdmin tags an operator-initiated refund in the ledger, distinct from a rail's own refund
+// (robokassa / vk / telegram). The refund idempotency key (providerAdmin, order id) allows exactly
+// one manual full refund per order.
+const providerAdmin = "admin"
+
+// RefundOrderFull refunds a paid order in full at the operator's request: it revokes the funded
+// chips best-effort (floored at 0, never negative — D27), records a refund ledger row, and is
+// idempotent (a second call reports AlreadyRefunded). The operator performs the actual money refund
+// on the rail (Robokassa cabinet / VK support / Telegram refundStarPayment); this records it.
+func (s *Service) RefundOrderFull(ctx context.Context, orderID uuid.UUID) (RefundOutcome, error) {
+	o, err := s.store.orderByID(ctx, orderID)
+	if err != nil {
+		return RefundOutcome{}, err
+	}
+	refunded, err := MoneyFromMinor(o.expectedAmount, Currency(o.currency))
+	if err != nil {
+		return RefundOutcome{}, err
+	}
+	return s.store.refund(ctx, orderID, providerAdmin, orderID.String(), refunded, s.clock())
+}
+
+// LedgerExportRow is one append-only ledger row for the tax / reconciliation export, carrying the
+// account it belongs to alongside the entry fields.
+type LedgerExportRow struct {
+	AccountID string
+	LedgerEntry
+}
+
+// LedgerExport reads the entire append-only ledger (all accounts, newest first) for a CSV/JSON
+// export — tax reporting and future rail reconciliation. Uncached, admin-only.
+func (s *Service) LedgerExport(ctx context.Context) ([]LedgerExportRow, error) {
+	return s.store.allLedger(ctx)
+}