feat: add core health endpoint and polls listing API

[h0pers]

Summary

• Move health check endpoint from polls app to a new core/api module for better separation of concerns
• Refactor polls API from a health check view to a proper polls listing endpoint with PollSerializer
• Add step names to pre-commit GitHub Actions workflow for better readability
• Minor style and formatting fixes across docker-compose and CI files

Test plan

• Verify GET /api/health/ returns {"status": "ok"}
• Verify GET /api/polls/ returns serialized poll data
• Confirm OpenAPI schema reflects updated endpoints and tags
• Verify pre-commit CI workflow runs successfully