feat: Migrate old events and gallery images to persistent storage

- Add migration script to convert and copy images
- Include 7 events (Karaoke, Pub Quiz, etc.) in WebP format
- Include 9 gallery images in WebP format
- Update .gitignore to allow images in data/images/
- Add migration documentation in MIGRATION_README.md

Images are stored in backend/data/images/ which maps to
the persistent Fly.io volume at /app/data/

backend/.gitignore

@@ -4,7 +4,9 @@ dist
 *.log
 .DS_Store
 /tmp
-/data
-*.db
-*.db-wal
-*.db-shm
+/data/*.db
+/data/*.db-wal
+/data/*.db-shm
+/data/workspace
+# Allow images to be committed
+!/data/images