V50: Add access_type to storage buckets
Adds access_type field to buckets for controlling how files are served:
- "public" - redirect to public URL (default, works with public S3 buckets)
- "private" - proxy through server (for ACL-protected buckets)
- "signed" - presigned URLs (future implementation)
Changes
- Adds access_type VARCHAR column to phoenix_kit_buckets with default "public"