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/package.json

@@ -9,7 +9,8 @@
     "start": "node dist/index.js",
     "db:generate": "drizzle-kit generate",
     "db:migrate": "drizzle-kit migrate",
-    "db:studio": "drizzle-kit studio"
+    "db:studio": "drizzle-kit studio",
+    "migrate:old-data": "tsx src/scripts/migrate-old-data.ts"
   },
   "dependencies": {
     "@fastify/cookie": "^9.3.1",