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/handlers/admin.go

@@ -3,6 +3,7 @@ package handlers
 import (
 	"crypto/rand"
 	"encoding/hex"
+	"errors"
 	"net/http"
 	"regexp"
 	"strconv"
@@ -44,6 +45,45 @@ type clientInput struct {
 	IsActive             *bool    `json:"is_active"`
 }
 
+// ── Global ZarinPal settings (admin-editable) ────────────────────────────────
+
+func (h *AdminHandler) GetSettings(c *gin.Context) {
+	s, err := h.store.GetSettings(c.Request.Context())
+	if err != nil {
+		if errors.Is(err, db.ErrNotFound) {
+			// Row missing (table exists) — return sane defaults so the form renders.
+			c.JSON(http.StatusOK, models.Settings{ZarinPalSandbox: true, ZarinPalAmountUnit: "rial"})
+			return
+		}
+		// Most likely the table doesn't exist yet — tell the admin to run migration 32.
+		c.JSON(http.StatusInternalServerError, models.APIError{Code: "db_error", Message: err.Error()})
+		return
+	}
+	c.JSON(http.StatusOK, s)
+}
+
+func (h *AdminHandler) UpdateSettings(c *gin.Context) {
+	var in struct {
+		ZarinPalMerchantID string `json:"zarinpal_merchant_id"`
+		ZarinPalSandbox    bool   `json:"zarinpal_sandbox"`
+		ZarinPalAmountUnit string `json:"zarinpal_amount_unit"`
+	}
+	if err := c.ShouldBindJSON(&in); err != nil {
+		c.JSON(http.StatusBadRequest, models.APIError{Code: "bad_request", Message: "invalid body"})
+		return
+	}
+	unit := strings.ToLower(strings.TrimSpace(in.ZarinPalAmountUnit))
+	if unit != "toman" {
+		unit = "rial"
+	}
+	s, err := h.store.UpdateSettings(c.Request.Context(), strings.TrimSpace(in.ZarinPalMerchantID), in.ZarinPalSandbox, unit)
+	if err != nil {
+		c.JSON(http.StatusInternalServerError, models.APIError{Code: "db_error", Message: err.Error()})
+		return
+	}
+	c.JSON(http.StatusOK, s)
+}
+
 func (h *AdminHandler) List(c *gin.Context) {
 	clients, err := h.store.ListClientApps(c.Request.Context())
 	if err != nil {