### Description Implement the FirebaseGateway that encapsulates all Firestore communication behind the logic/api interfaces. Requirements covered: Architecture: SessionGateway, LeaderboardRepository interfaces in logic/api Architecture: FirebaseGateway in infra/persistence Acceptance criteria: SessionGateway interface in logic/api LeaderboardRepository interface in logic/api FirebaseGateway implements both using Firestore Real-time listeners in infra/sync for state changes ### Related requirement _No response_ ### Architecture layer None