Session: immutable holder of token, owning User, createdAt,
expiresAt. isExpired(now) compares >= expiresAt. SessionModel
keys on token (string primary, non-incrementing). migration adds
sessions table with foreign user_id (cascade on user delete) and
indexed expires_at for cleanup queries. EloquentSessionRepository
takes UserRepository to rehydrate the owning User on findByToken;
sessions for deleted users return null. FakeSessionRepository
mirrors with an in-memory map keyed by token, defensive copies on
read.
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