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/mirror-nginx-flatrender.conf

@@ -29,7 +29,7 @@
 
 server {
     listen 80;
-    server_name flatrender.ir www.flatrender.ir api.flatrender.ir storage.flatrender.ir;
+    server_name flatrender.ir www.flatrender.ir api.flatrender.ir storage.flatrender.ir pay.flatrender.ir;
     return 301 https://$host$request_uri;
 }
 
@@ -87,3 +87,23 @@ server {
         proxy_set_header X-Forwarded-Proto $scheme;
     }
 }
+
+# ── Payment broker (→ PAY_PORT) — pay.flatrender.ir ───────────────────────
+# The single ZarinPal-verified callback domain. ZarinPal redirects users to
+# pay.flatrender.ir/callback/zarinpal; the broker then bounces them to the
+# originating site's return_url. Must NOT be cached by any upstream CDN.
+server {
+    listen 443 ssl; http2 on;
+    server_name pay.flatrender.ir;
+    client_max_body_size 5m;
+    ssl_certificate     /etc/ssl/soroushasadi/flatrender/fullchain.pem;
+    ssl_certificate_key /etc/ssl/soroushasadi/flatrender/privateKey.pem;
+    location / {
+        proxy_pass http://171.22.25.73:1607;
+        proxy_http_version 1.1;
+        proxy_set_header Host $host;
+        proxy_set_header X-Real-IP $remote_addr;
+        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
+        proxy_set_header X-Forwarded-Proto $scheme;
+    }
+}