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/store_statement.go

@@ -77,6 +77,36 @@ func (s *Store) accountStatement(ctx context.Context, accountID uuid.UUID) (Stat
 	return out, nil
 }
 
+// allLedger reads the entire append-only ledger (all accounts, newest first) for the admin export.
+func (s *Store) allLedger(ctx context.Context) ([]LedgerExportRow, error) {
+	var rows []model.Ledger
+	if err := postgres.SELECT(table.Ledger.AllColumns).
+		FROM(table.Ledger).
+		ORDER_BY(table.Ledger.CreatedAt.DESC()).
+		QueryContext(ctx, s.db, &rows); err != nil {
+		return nil, fmt.Errorf("payments: export ledger: %w", err)
+	}
+	out := make([]LedgerExportRow, 0, len(rows))
+	for _, r := range rows {
+		out = append(out, LedgerExportRow{
+			AccountID: r.AccountID.String(),
+			LedgerEntry: LedgerEntry{
+				Kind:              r.Kind,
+				Source:            derefStr(r.Source),
+				Origin:            derefStr(r.Origin),
+				ChipsDelta:        int(r.ChipsDelta),
+				ProductID:         derefUUID(r.ProductID),
+				OrderID:           derefUUID(r.OrderID),
+				Provider:          derefStr(r.Provider),
+				ProviderPaymentID: derefStr(r.ProviderPaymentID),
+				Snapshot:          derefStr(r.Snapshot),
+				CreatedAt:         r.CreatedAt,
+			},
+		})
+	}
+	return out, nil
+}
+
 // derefStr returns the pointed-to string, or "" when the pointer is nil (a NULL column).
 func derefStr(p *string) string {
 	if p == nil {