feat(payment): admin-editable ZarinPal settings + in-panel test payment

Lets the broker's ZarinPal merchant / sandbox / amount-unit be set from
Admin → درگاه پرداخت (persisted in payment.settings) instead of env +
redeploy, and adds a per-app "test payment" button that mints a real
ZarinPal StartPay link straight from the panel — no site wiring needed.

- migration 33_payment_settings.sql: singleton payment.settings + a
  transactions.is_test column. (33, not 32 — 32 is content_render_engine.)
- broker read-path precedence: per-client override > DB settings > env.
- POST /v1/admin/clients/:id/test-payment + GET/PUT /v1/admin/settings.
- admin UI: «تنظیمات زرین‌پال» tab + «پرداخت آزمایشی» button.

Adversarial-review fixes (2 confirmed HIGH):
- do NOT pre-seed the settings row — a seeded sandbox=TRUE default would
  override a production ZARINPAL_SANDBOX=false env and silently route real
  payments to sandbox.zarinpal.com until an admin untouched the toggle.
  No row → env governs until an admin saves.
- test transactions are tagged is_test and the webhook dispatcher skips
  them, so an admin smoke-test can never notify (or credit) a real client,
  regardless of metadata. Broker-authoritative, not consumer-dependent.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

services/payment/internal/db/db.go

@@ -22,6 +22,35 @@ func NewStore(pool *pgxpool.Pool) *Store { return &Store{pool: pool} }
 
 func (s *Store) Ping(ctx context.Context) error { return s.pool.Ping(ctx) }
 
+// ── Global settings (singleton row id=1) ─────────────────────────────────────
+
+func (s *Store) GetSettings(ctx context.Context) (*models.Settings, error) {
+	var st models.Settings
+	err := s.pool.QueryRow(ctx, `
+		SELECT zarinpal_merchant_id, zarinpal_sandbox, zarinpal_amount_unit, updated_at
+		FROM payment.settings WHERE id = 1`).Scan(
+		&st.ZarinPalMerchantID, &st.ZarinPalSandbox, &st.ZarinPalAmountUnit, &st.UpdatedAt)
+	if errors.Is(err, pgx.ErrNoRows) {
+		return nil, ErrNotFound
+	}
+	return &st, err
+}
+
+func (s *Store) UpdateSettings(ctx context.Context, merchant string, sandbox bool, unit string) (*models.Settings, error) {
+	var st models.Settings
+	err := s.pool.QueryRow(ctx, `
+		INSERT INTO payment.settings (id, zarinpal_merchant_id, zarinpal_sandbox, zarinpal_amount_unit)
+		VALUES (1, $1, $2, $3)
+		ON CONFLICT (id) DO UPDATE SET
+		  zarinpal_merchant_id = EXCLUDED.zarinpal_merchant_id,
+		  zarinpal_sandbox     = EXCLUDED.zarinpal_sandbox,
+		  zarinpal_amount_unit = EXCLUDED.zarinpal_amount_unit
+		RETURNING zarinpal_merchant_id, zarinpal_sandbox, zarinpal_amount_unit, updated_at`,
+		merchant, sandbox, unit).Scan(
+		&st.ZarinPalMerchantID, &st.ZarinPalSandbox, &st.ZarinPalAmountUnit, &st.UpdatedAt)
+	return &st, err
+}
+
 // ── Client apps ───────────────────────────────────────────────────────────────
 
 const clientCols = `id, tenant_id, name, slug, api_key, secret,
@@ -133,7 +162,7 @@ func (s *Store) DeleteClientApp(ctx context.Context, id uuid.UUID) error {
 const txnCols = `id, client_app_id, status, gateway, amount_rial, currency, description,
 	client_ref, return_url, metadata, payer_mobile, payer_email,
 	authority, ref_id, card_pan, fee_rial, gateway_response, failure_reason,
-	paid_at, failed_at, expires_at, created_at, updated_at`
+	paid_at, failed_at, expires_at, created_at, updated_at, is_test`
 
 func scanTxn(row pgx.Row) (*models.Transaction, error) {
 	var t models.Transaction
@@ -142,7 +171,7 @@ func scanTxn(row pgx.Row) (*models.Transaction, error) {
 		&t.ID, &t.ClientAppID, &t.Status, &t.Gateway, &t.AmountRial, &t.Currency, &t.Description,
 		&t.ClientRef, &t.ReturnURL, &meta, &t.PayerMobile, &t.PayerEmail,
 		&t.Authority, &t.RefID, &t.CardPan, &t.FeeRial, &gwResp, &t.FailureReason,
-		&t.PaidAt, &t.FailedAt, &t.ExpiresAt, &t.CreatedAt, &t.UpdatedAt,
+		&t.PaidAt, &t.FailedAt, &t.ExpiresAt, &t.CreatedAt, &t.UpdatedAt, &t.IsTest,
 	); err != nil {
 		return nil, err
 	}
@@ -159,11 +188,11 @@ func (s *Store) CreateTransaction(ctx context.Context, t *models.Transaction) (*
 	row := s.pool.QueryRow(ctx, `
 		INSERT INTO payment.transactions
 		  (client_app_id, status, gateway, amount_rial, currency, description,
-		   client_ref, return_url, metadata, payer_mobile, payer_email, expires_at)
-		VALUES ($1,$2,$3,$4,$5,$6,$7,$8,$9,$10,$11,$12)
+		   client_ref, return_url, metadata, payer_mobile, payer_email, expires_at, is_test)
+		VALUES ($1,$2,$3,$4,$5,$6,$7,$8,$9,$10,$11,$12,$13)
 		RETURNING `+txnCols,
 		t.ClientAppID, t.Status, t.Gateway, t.AmountRial, t.Currency, t.Description,
-		t.ClientRef, t.ReturnURL, meta, t.PayerMobile, t.PayerEmail, t.ExpiresAt)
+		t.ClientRef, t.ReturnURL, meta, t.PayerMobile, t.PayerEmail, t.ExpiresAt, t.IsTest)
 	return scanTxn(row)
 }