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/twitchyliquid64/golang-asm/unsafeheader/unsafeheader.go

@@ -0,0 +1,37 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package unsafeheader contains header declarations for the Go runtime's slice
+// and string implementations.
+//
+// This package allows packages that cannot import "reflect" to use types that
+// are tested to be equivalent to reflect.SliceHeader and reflect.StringHeader.
+package unsafeheader
+
+import (
+	"unsafe"
+)
+
+// Slice is the runtime representation of a slice.
+// It cannot be used safely or portably and its representation may
+// change in a later release.
+//
+// Unlike reflect.SliceHeader, its Data field is sufficient to guarantee the
+// data it references will not be garbage collected.
+type Slice struct {
+	Data unsafe.Pointer
+	Len  int
+	Cap  int
+}
+
+// String is the runtime representation of a string.
+// It cannot be used safely or portably and its representation may
+// change in a later release.
+//
+// Unlike reflect.StringHeader, its Data field is sufficient to guarantee the
+// data it references will not be garbage collected.
+type String struct {
+	Data unsafe.Pointer
+	Len  int
+}