- Update Firebase service to use clean GCS URLs instead of MediaLink
- Set ACL to make uploaded files publicly accessible
- Use slug as image filename to prevent overwrites
- Add unique constraints on slug columns in books and stationery tables
- Add slug existence checks before upload (409 Conflict if duplicate)
- Update storage paths: /jd-bookshop/books and /jd-bookshop/stationery
- Remove year/month from storage paths as requested
- Construct URLs: https://storage.googleapis.com/{bucket}/{encoded-path}
Changes:
- firebase.go: Set ACL, construct clean URLs using url.PathEscape
- book.go & stationery.go: Check slug before upload, use correct paths
- book_service.go & stationery_service.go: Add slug existence check functions
- migration: Add UNIQUE constraints on slug columns
