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/goccy/go-json/Makefile

@@ -0,0 +1,39 @@
+PKG := github.com/goccy/go-json
+
+BIN_DIR := $(CURDIR)/bin
+PKGS := $(shell go list ./... | grep -v internal/cmd|grep -v test)
+COVER_PKGS := $(foreach pkg,$(PKGS),$(subst $(PKG),.,$(pkg)))
+
+COMMA := ,
+EMPTY :=
+SPACE := $(EMPTY) $(EMPTY)
+COVERPKG_OPT := $(subst $(SPACE),$(COMMA),$(COVER_PKGS))
+
+$(BIN_DIR):
+	@mkdir -p $(BIN_DIR)
+
+.PHONY: cover
+cover:
+	go test -coverpkg=$(COVERPKG_OPT) -coverprofile=cover.out ./...
+
+.PHONY: cover-html
+cover-html: cover
+	go tool cover -html=cover.out
+
+.PHONY: lint
+lint: golangci-lint
+	$(BIN_DIR)/golangci-lint run
+
+golangci-lint: | $(BIN_DIR)
+	@{ \
+		set -e; \
+		GOLANGCI_LINT_TMP_DIR=$$(mktemp -d); \
+		cd $$GOLANGCI_LINT_TMP_DIR; \
+		go mod init tmp; \
+		GOBIN=$(BIN_DIR) go install github.com/golangci/golangci-lint/cmd/golangci-lint@v1.48.0; \
+		rm -rf $$GOLANGCI_LINT_TMP_DIR; \
+	}
+
+.PHONY: generate
+generate:
+	go generate ./internal/...