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/vendor/github.com/gin-gonic/gin/fs.go

@@ -0,0 +1,45 @@
+// Copyright 2017 Manu Martinez-Almeida. All rights reserved.
+// Use of this source code is governed by a MIT style
+// license that can be found in the LICENSE file.
+
+package gin
+
+import (
+	"net/http"
+	"os"
+)
+
+type onlyFilesFS struct {
+	fs http.FileSystem
+}
+
+type neuteredReaddirFile struct {
+	http.File
+}
+
+// Dir returns a http.FileSystem that can be used by http.FileServer(). It is used internally
+// in router.Static().
+// if listDirectory == true, then it works the same as http.Dir() otherwise it returns
+// a filesystem that prevents http.FileServer() to list the directory files.
+func Dir(root string, listDirectory bool) http.FileSystem {
+	fs := http.Dir(root)
+	if listDirectory {
+		return fs
+	}
+	return &onlyFilesFS{fs}
+}
+
+// Open conforms to http.Filesystem.
+func (fs onlyFilesFS) Open(name string) (http.File, error) {
+	f, err := fs.fs.Open(name)
+	if err != nil {
+		return nil, err
+	}
+	return neuteredReaddirFile{f}, nil
+}
+
+// Readdir overrides the http.File default implementation.
+func (f neuteredReaddirFile) Readdir(_ int) ([]os.FileInfo, error) {
+	// this disables directory listing
+	return nil, nil
+}