Feat: Genre system with per-genre daily puzzles

prisma/schema.prisma

@@ -18,11 +18,23 @@ model Song {
   coverImage String?  // Filename in public/uploads/covers
   createdAt DateTime @default(now())
   puzzles   DailyPuzzle[]
+  genres    Genre[]
+}
+
+model Genre {
+  id    Int    @id @default(autoincrement())
+  name  String @unique
+  songs Song[]
+  dailyPuzzles DailyPuzzle[]
 }
 
 model DailyPuzzle {
   id        Int      @id @default(autoincrement())
-  date      String   @unique // Format: YYYY-MM-DD
+  date      String   // Format: YYYY-MM-DD
   songId    Int
   song      Song     @relation(fields: [songId], references: [id])
+  genreId   Int?
+  genre     Genre?   @relation(fields: [genreId], references: [id])
+
+  @@unique([date, genreId]) // Unique puzzle per date per genre (null genreId = global puzzle)
 }