fix duplicate miner storage

[anderdc]

Summary

bug: duplicate miners showed up on leaderboard in UI

• resolved by cleaning up stale miner evaluations in storage.py

bug: cached miner responses still were being stored and receiving a new updated_at value, causing confusion in dashboard

• resolved by not re-storing miner evaluation if it was a cached evaluation

Related Issues

None

Type of Change

• Bug fix
• New feature
• Refactor
• Documentation
• Other (describe below)

Testing

• Tests added/updated
• Manually tested

Checklist

• Code follows project style guidelines
• Self-review completed
• Changes are documented (if applicable)