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/models/models.go

@@ -13,6 +13,14 @@ type APIError struct {
 	Message string `json:"message"`
 }
 
+// Settings is the singleton global broker config (admin-editable ZarinPal default).
+type Settings struct {
+	ZarinPalMerchantID string    `json:"zarinpal_merchant_id"`
+	ZarinPalSandbox    bool      `json:"zarinpal_sandbox"`
+	ZarinPalAmountUnit string    `json:"zarinpal_amount_unit"` // "rial" | "toman"
+	UpdatedAt          time.Time `json:"updated_at"`
+}
+
 // ── Client apps (tenants of the broker — each site that pays through it) ───────
 
 type ClientApp struct {
@@ -39,6 +47,7 @@ type Transaction struct {
 	ClientSlug      string          `json:"client_slug,omitempty"` // joined for admin views
 	Status          string          `json:"status"`
 	Gateway         string          `json:"gateway"`
+	IsTest          bool            `json:"is_test"`
 	AmountRial      int64           `json:"amount_rial"`
 	Currency        string          `json:"currency"`
 	Description     *string         `json:"description,omitempty"`