Add backend response validation coverage for admin routes

[mimijuwonlo-commits]

Area

Backend API correctness

Problem

Response validation exists, but privileged admin routes should be included so schema drift does not break dashboards or incident tooling.

Acceptance Criteria

• Enable or verify response validation on admin routes
• Add tests for stats, audit, and payment admin responses
• Document any intentionally unvalidated streaming routes
• Update OpenAPI if schemas are missing

Validation

Run backend admin route tests and response validation tests.