Create events table and CRUD

[jedmund]

Summary

• Add slug column to events, auto-generated from name
• Remove banner_image column, derive path from slug instead
• Blueprint returns computed banner_image path
• upload_banner uses slug for S3 key

Test plan

• bundle exec rspec passes (33 examples, 0 failures)
• Create event, verify slug auto-generated
• Upload banner, verify S3 path uses slug