Add payments-mcp (Layer 3 MCP surface), single-route pattern from the start

list-payments/get-payment (all 5 roles) and create-payment (admin
only — matches PaymentPermission's actual write-access logic, not
guessed). One KongRoute for GET/POST/DELETE, one ai-mcp-proxy
binding — avoids the duplicate-tool-name bug found and fixed on
customers-mcp (STATUS.md section 18), built correctly here instead
of repeating the two-route mistake.

Keycloak (local): mcp-payments-audience client scope + mcp-payments
resource client, attached to librechat as an optional scope, no
dedicated protocol mappers on the client itself.

kong/mcp/openid-connect.yaml

@@ -0,0 +1,27 @@
+apiVersion: configuration.konghq.com/v1
+kind: KongPlugin
+metadata:
+  name: ${NAME}-payments-mcp-openid-connect
+  namespace: ${NAMESPACE}
+plugin: openid-connect
+config:
+  issuer: ${OIDC_ISSUER}
+  audience_required: ${API_AUDIENCE}
+  auth_methods:
+    - bearer
+  display_errors: false
+  ssl_verify: true
+  consumer_groups_claim:
+    - realm_access
+    - roles
+  client_id:
+    - ${TOKEN_EXCHANGE_CLIENT_ID}
+  client_secret:
+    - ${TOKEN_EXCHANGE_CLIENT_SECRET}
+  token_exchange:
+    subject_token_issuers:
+      - issuer: ${OIDC_ISSUER}
+        conditions: {}
+    request:
+      audience: ${API_AUDIENCE}
+      scopes: []