[Feature] Firebase gateway (infra layer)

[milospi]

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