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>

deploy/ENV_FILE.production.example

@@ -22,6 +22,7 @@ EDGE_BIND=0.0.0.0
 # nginx-facing host ports (must be free on 171.22.25.73 — :3000 is Gitea, avoid it).
 FRONTEND_PORT=1600
 GATEWAY_PORT=1605
+PAY_PORT=1607
 MINIO_PORT=1610
 MINIO_CONSOLE_PORT=1611
 
@@ -58,10 +59,20 @@ MINIO_HOST_USE_SSL=true
 RENDER_DEV_WORKER=false
 RENDER_DEV_SNAPSHOTS=false
 
+# ── Payment broker (pay.flatrender.ir) ───────────────────────────────────────
+# Standalone ZarinPal gateway shared by FlatRender + meezi.ir + bargevasat.ir.
+# ZARINPAL_MERCHANT_ID below is the SHARED merchant (verified domain = pay.flatrender.ir).
+PAY_PUBLIC_URL=https://pay.flatrender.ir
+# Unit ZarinPal expects in `amount`: "rial" (official v4) or "toman".
+# ⚠️ Your identity service historically sends Toman — confirm with one sandbox
+# payment which unit YOUR merchant settles in, then set this to match.
+ZARINPAL_AMOUNT_UNIT=rial
+
 # ── Payments (fill the providers you use; leave others blank) ────────────────
 STRIPE_SECRET_KEY=
 STRIPE_WEBHOOK_SECRET=
 STRIPE_PUBLISHABLE_KEY=
+# Shared ZarinPal merchant — used by BOTH the identity service and the pay broker.
 ZARINPAL_MERCHANT_ID=
 ZARINPAL_CALLBACK_URL=https://api.flatrender.ir/v1/payments/callback/zarinpal
 ZARINPAL_SANDBOX=false