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/cloudwego/base64x/faststr.go

@@ -0,0 +1,51 @@
+/*
+ * Copyright 2024 CloudWeGo Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package base64x
+
+import (
+    `reflect`
+    `unsafe`
+)
+
+func mem2str(v []byte) (s string) {
+    (*reflect.StringHeader)(unsafe.Pointer(&s)).Len  = (*reflect.SliceHeader)(unsafe.Pointer(&v)).Len
+    (*reflect.StringHeader)(unsafe.Pointer(&s)).Data = (*reflect.SliceHeader)(unsafe.Pointer(&v)).Data
+    return
+}
+
+func str2mem(s string) (v []byte) {
+    (*reflect.SliceHeader)(unsafe.Pointer(&v)).Cap  = (*reflect.StringHeader)(unsafe.Pointer(&s)).Len
+    (*reflect.SliceHeader)(unsafe.Pointer(&v)).Len  = (*reflect.StringHeader)(unsafe.Pointer(&s)).Len
+    (*reflect.SliceHeader)(unsafe.Pointer(&v)).Data = (*reflect.StringHeader)(unsafe.Pointer(&s)).Data
+    return
+}
+
+func mem2addr(v []byte) unsafe.Pointer {
+    return *(*unsafe.Pointer)(unsafe.Pointer(&v))
+}
+
+// NoEscape hides a pointer from escape analysis. NoEscape is
+// the identity function but escape analysis doesn't think the
+// output depends on the input. NoEscape is inlined and currently
+// compiles down to zero instructions.
+// USE CAREFULLY!
+//go:nosplit
+//goland:noinspection GoVetUnsafePointer
+func noEscape(p unsafe.Pointer) unsafe.Pointer {
+    x := uintptr(p)
+    return unsafe.Pointer(x ^ 0)
+}