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/migrations/20251201190234_add_player_state/migration.sql

@@ -0,0 +1,16 @@
+-- CreateTable
+CREATE TABLE "PlayerState" (
+    "id" INTEGER NOT NULL PRIMARY KEY AUTOINCREMENT,
+    "identifier" TEXT NOT NULL,
+    "genreKey" TEXT NOT NULL,
+    "gameState" TEXT NOT NULL,
+    "statistics" TEXT NOT NULL,
+    "lastPlayed" DATETIME NOT NULL,
+    "createdAt" DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP
+);
+
+-- CreateIndex
+CREATE INDEX "PlayerState_identifier_idx" ON "PlayerState"("identifier");
+
+-- CreateIndex
+CREATE UNIQUE INDEX "PlayerState_identifier_genreKey_key" ON "PlayerState"("identifier", "genreKey");