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/.golangci.yml

@@ -0,0 +1,57 @@
+run:
+  timeout: 5m
+linters:
+  enable:
+    - asciicheck
+    - dogsled
+    - durationcheck
+    - errcheck
+    - errorlint
+    - exportloopref
+    - gci
+    - gofmt
+    - goimports
+    - gosec
+    - misspell
+    - nakedret
+    - nilerr
+    - nolintlint
+    - revive
+    - wastedassign
+
+linters-settings:
+  gosec:
+    # To select a subset of rules to run.
+    # Available rules: https://github.com/securego/gosec#available-rules
+    # Default: [] - means include all rules
+    includes:
+      - G102
+      - G106
+      - G108
+      - G109
+      - G111
+      - G112
+      - G201
+      - G203
+
+issues:
+  exclude-rules:
+    - linters:
+        - structcheck
+        - unused
+      text: "`data` is unused"
+    - linters:
+        - staticcheck
+      text: "SA1019:"
+    - linters:
+        - revive
+      text: "var-naming:"
+    - linters:
+        - revive
+      text: "exported:"
+    - path: _test\.go
+      linters:
+        - gosec # security is not make sense in tests
+    - linters:
+        - revive
+      path: _test\.go