Backend: Validation Evidence

Backend behavior changes need validation evidence.

Scope:
- Enforce when PR changes backend behavior (not purely comments/formatting/renames).

Rules:
1) PR description should include a short validation section with exact commands or concrete manual validation steps.
2) Include pass/fail outcome summary for those steps.
3) For behavior-changing backend API logic, prefer automated tests when practical.

Review behavior:
- Return NOT APPROVED only when behavior-changing backend work has no credible validation evidence.
- Do not block clearly non-behavioral changes.