Backend: Add routes-f schema validator #304
Description
Summary
Implement a Routes-F request schema validator using a lightweight schema definition.
Requirements
- Define schemas per endpoint in routes-f.
- Reject unknown fields by default.
- Provide a typed result for handlers.
Notes
- Avoid heavy validation libs unless already used.
Acceptance Criteria
- At least 3 routes use the validator.
- Tests cover unknown field rejection.