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/CONTRIBUTING.md

@@ -0,0 +1,55 @@
+# How to Contribute
+
+## Your First Pull Request
+We use github for our codebase. You can start by reading [How To Pull Request](https://docs.github.com/en/github/collaborating-with-issues-and-pull-requests/about-pull-requests).
+
+## Branch Organization
+We use [git-flow](https://nvie.com/posts/a-successful-git-branching-model/) as our branch organization, as known as [FDD](https://en.wikipedia.org/wiki/Feature-driven_development)
+
+## Bugs
+### 1. How to Find Known Issues
+We are using [Github Issues](https://github.com/cloudwego/kitex/issues) for our public bugs. We keep a close eye on this and try to make it clear when we have an internal fix in progress. Before filing a new task, try to make sure your problem doesn’t already exist.
+
+### 2. Reporting New Issues
+Providing a reduced test code is a recommended way for reporting issues. Then can placed in:
+- Just in issues
+- [Golang Playground](https://play.golang.org/)
+
+### 3. Security Bugs
+Please do not report the safe disclosure of bugs to public issues. Contact us by [Support Email](mailto:conduct@cloudwego.io)
+
+## How to Get in Touch
+- [Email](mailto:conduct@cloudwego.io)
+
+## Submit a Pull Request
+Before you submit your Pull Request (PR) consider the following guidelines:
+1. Search [GitHub](https://github.com/cloudwego/kitex/pulls) for an open or closed PR that relates to your submission. You don't want to duplicate existing efforts.
+2. Be sure that an issue describes the problem you're fixing, or documents the design for the feature you'd like to add. Discussing the design upfront helps to ensure that we're ready to accept your work.
+3. [Fork](https://docs.github.com/en/github/getting-started-with-github/fork-a-repo) the cloudwego/kitex repo.
+4. In your forked repository, make your changes in a new git branch:
+    ```
+    git checkout -b my-fix-branch develop
+    ```
+5. Create your patch, including appropriate test cases.
+6. Follow our [Style Guides](#code-style-guides).
+7. Commit your changes using a descriptive commit message that follows [AngularJS Git Commit Message Conventions](https://docs.google.com/document/d/1QrDFcIiPjSLDn3EL15IJygNPiHORgU1_OOAqWjiDU5Y/edit).
+   Adherence to these conventions is necessary because release notes are automatically generated from these messages.
+8. Push your branch to GitHub:
+    ```
+    git push origin my-fix-branch
+    ```
+9. In GitHub, send a pull request to `kitex:develop`
+
+## Contribution Prerequisites
+- Our development environment keeps up with [Go Official](https://golang.org/project/).
+- You need fully checking with lint tools before submit your pull request. [gofmt](https://golang.org/pkg/cmd/gofmt/) and [golangci-lint](https://github.com/golangci/golangci-lint)
+- You are familiar with [Github](https://github.com)
+- Maybe you need familiar with [Actions](https://github.com/features/actions)(our default workflow tool).
+
+## Code Style Guides
+Also see [Pingcap General advice](https://pingcap.github.io/style-guide/general.html).
+
+Good resources:
+- [Effective Go](https://golang.org/doc/effective_go)
+- [Go Code Review Comments](https://github.com/golang/go/wiki/CodeReviewComments)
+- [Uber Go Style Guide](https://github.com/uber-go/guide/blob/master/style.md)