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/cmd/server/main.go

@@ -67,12 +67,15 @@ func main() {
 	// ── Admin API (FlatRender admin JWT) ─────────────────────────────────────
 	admin := v1.Group("/admin", middleware.JWTAuth(cfg.JWTSecret), middleware.RequireAdmin())
 	{
+		admin.GET("/settings", adminH.GetSettings)
+		admin.PUT("/settings", adminH.UpdateSettings)
 		admin.GET("/clients", adminH.List)
 		admin.POST("/clients", adminH.Create)
 		admin.GET("/clients/:id", adminH.Get)
 		admin.PUT("/clients/:id", adminH.Update)
 		admin.DELETE("/clients/:id", adminH.Delete)
 		admin.POST("/clients/:id/rotate-secret", adminH.RotateSecret)
+		admin.POST("/clients/:id/test-payment", payH.AdminTest)
 		admin.GET("/transactions", adminH.ListTransactions)
 	}