feat: Add cover art support and auto-migration

- Extract cover art from MP3s during upload
- Display cover art in game result screens (win/loss)
- Add coverImage field to Song model
- Add migration script to backfill covers for existing songs
- Configure Docker to run migration script on startup

prisma/schema.prisma

@@ -15,6 +15,7 @@ model Song {
   title     String
   artist    String
   filename  String   // Filename in public/uploads
+  coverImage String?  // Filename in public/uploads/covers
   createdAt DateTime @default(now())
   puzzles   DailyPuzzle[]
 }