feat: Add backend storage for cross-domain player state synchronization

- Add PlayerState model to database schema for storing game states
- Create player identifier system (UUID-based) for cross-domain sync
- Implement API endpoints for loading/saving player states
- Refactor gameState hook to use backend storage with localStorage fallback
- Support synchronization between hoerdle.de and hördle.de
- Migration automatically runs on Docker container start

prisma/schema.prisma

@@ -88,3 +88,16 @@ model News {
   
   @@index([publishedAt])
 }
+
+model PlayerState {
+  id          Int      @id @default(autoincrement())
+  identifier  String   // UUID des Spielers (für Cross-Domain-Sync)
+  genreKey    String   // Genre-Name oder "global" oder "special:<name>"
+  gameState   String   // JSON-String des GameState
+  statistics  String   // JSON-String der Statistics
+  lastPlayed  DateTime @updatedAt
+  createdAt   DateTime @default(now())
+  
+  @@unique([identifier, genreKey])
+  @@index([identifier])
+}