test: mutation testing for user rating module

[mostronatorcoder]

Context

Follow-up from #618. Implement mutation testing for user rating — impacts reputation and trust between traders.

Scope

• src/app/rate_user.rs — User rating logic

Goal

• Run cargo mutants --file src/app/rate_user.rs
• Kill surviving mutants
• Target: >70% mutation score

Priority Mutants to Kill

• Rating validation (bounds, allowed values)
• Rating calculation/aggregation
• Permission checks (who can rate whom)
• Double-rating prevention

Acceptance Criteria

• Baseline mutation report
• Critical mutants killed
• Mutation score documented in PR

Ref: #618