feat: add creator list validation, serializer, and cache-control

[xqcxx]

Summary

• add a new creators module with a public GET /api/v1/creators endpoint
• centralize creator list query validation and summary serialization for consistent list responses
• add explicit cache-control middleware and apply it to the public creator list route

Why

This groups creator listing concerns in one place so list responses are consistent, query validation is reusable, and public GET routes have intentional cache behavior.

Closes #13
Closes #19
Closes #21

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