Build payments-api REST layer (Layer 3), mirroring customers-api

6 routes over Payment, composite key (customernumber + checknumber),
proxying to classic-models-api. openid-connect with RFC 8693 token
exchange to classic-models-api audience, rate-limiting, proxy-cache,
audit-log — same structure as customers-api's REST layer.

Keycloak (local): payments-api-audience client scope + payments-api
resource client added, attached to kong-token-exchange-broker's
default scopes. No dedicated protocol mappers on payments-api
itself, same practice as customers-api/mcp-customers.

No MCP layer — same as customers-api, not needed yet.

dev/gitrepository.yaml

@@ -0,0 +1,12 @@
+apiVersion: source.toolkit.fluxcd.io/v1
+kind: GitRepository
+metadata:
+  name: payments-api-dev
+  namespace: payments
+spec:
+  interval: 5m
+  url: ssh://git@gitea.sttlab.eu:2222/stt/kong-payments-api.git
+  secretRef:
+    name: payments-api-git-auth
+  ref:
+    branch: main