Fix critical bug and deprecation warnings

- Fixed bug in games_router.py: player_result -> player_stmt in create_game
- Updated main.py lifespan from generator to @asynccontextmanager (FastAPI deprecation)
- security.py already uses datetime.now(timezone.utc) - no change needed
- All 28 tests pass

backend/app/routers/games_router.py

@@ -346,7 +346,7 @@ async def create_game(
     
     # Fetch players for response
     player_stmt = select(GamePlayer).where(GamePlayer.game_id == new_game.id)
-    player_result = await db.execute(player_result)
+    player_result = await db.execute(player_stmt)
     players = player_result.scalars().all()
     
     new_game.players = [GamePlayerResponse.model_validate(p) for p in players]