feat(payment): standalone ZarinPal broker on pay.flatrender.ir

A generic multi-client payment gateway so FlatRender, meezi.ir and
bargevasat.ir can all pay through ZarinPal's single verified callback
domain (pay.flatrender.ir).

New Go service services/payment (clones the notification skeleton +
vendored deps):
- migration 31_payment_broker.sql — `payment` schema: client_apps,
  transactions, webhook_deliveries.
- ZarinPal v4 client ported from the proven identity PaymentService
  (request.json -> StartPay -> verify.json; codes 100/101).
- client API: POST /v1/pay/request + /v1/pay/inquiry, authed by
  X-Api-Key + HMAC body signature; GET /callback/zarinpal (the single
  verified endpoint) verifies, then 302s the user back to the site's
  return_url (signed) and fires a signed, retried webhook.
- per-client ZarinPal merchant override (default = shared merchant);
  amount stored canonically in Rial, unit to ZarinPal env-configurable.
- admin API /v1/admin/* (FlatRender admin JWT): client-app CRUD +
  key issue/rotate + transactions list.

Deploy wiring: payment-svc in docker-compose.v2.yml (host port 1607),
pay.flatrender.ir server block in mirror-nginx conf, ENV_FILE +
README updates (cert SAN + manual migration note).

Admin UI: src/components/admin/PaymentsAdmin.tsx (client apps with
one-time key reveal + rotate, transactions table) + /admin/payments
page + nav link + fa/en strings; pay-admin proxy route to payment-svc.

Docs/SDK: deploy/PAYMENTS.md (integration contract) + deploy/sdk/flatpay.js
(zero-dep Node client + webhook verifier) for meezi/any site.

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

services/payment/internal/config/config.go

@@ -0,0 +1,54 @@
+package config
+
+import (
+	"os"
+	"strings"
+)
+
+// Config is the payment-broker runtime configuration, all from env.
+type Config struct {
+	DatabaseURL string
+	JWTSecret   string // verifies FlatRender admin JWTs for /v1/admin/* endpoints
+	Port        string
+
+	// PublicBaseURL is the broker's externally reachable base (e.g. https://pay.flatrender.ir).
+	// ZarinPal callbacks and the user redirect target are built from it.
+	PublicBaseURL string
+
+	// ── ZarinPal (shared default merchant) ──────────────────────────────────
+	// A client_app may override MerchantId/Sandbox; otherwise these defaults apply.
+	ZarinPalMerchantID string
+	ZarinPalSandbox    bool
+	// AmountUnit is the unit ZarinPal expects in the amount field: "rial" (official
+	// v4) or "toman". The broker always stores Rial canonically and converts here.
+	ZarinPalAmountUnit string
+}
+
+func getEnv(key, fallback string) string {
+	if v := os.Getenv(key); v != "" {
+		return v
+	}
+	return fallback
+}
+
+func Load() Config {
+	base := strings.TrimRight(getEnv("PUBLIC_BASE_URL", "http://localhost:8080"), "/")
+	unit := strings.ToLower(getEnv("ZARINPAL_AMOUNT_UNIT", "rial"))
+	if unit != "toman" {
+		unit = "rial"
+	}
+	return Config{
+		DatabaseURL:        getEnv("DATABASE_URL", "postgres://postgres:postgres@localhost:5432/flatrender?search_path=payment,public"),
+		JWTSecret:          getEnv("JWT_SECRET", "change-me"),
+		Port:               getEnv("PORT", "8080"),
+		PublicBaseURL:      base,
+		ZarinPalMerchantID: getEnv("ZARINPAL_MERCHANT_ID", ""),
+		ZarinPalSandbox:    getEnv("ZARINPAL_SANDBOX", "true") == "true",
+		ZarinPalAmountUnit: unit,
+	}
+}
+
+// CallbackURL is the single ZarinPal-verified callback endpoint on this broker.
+func (c Config) CallbackURL() string {
+	return c.PublicBaseURL + "/callback/zarinpal"
+}