add Session entity, persistence, fake

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.

backend/app/Auth/SessionRepository.php

@@ -0,0 +1,12 @@
+<?php
+
+namespace App\Auth;
+
+interface SessionRepository
+{
+    public function create(CreateSessionDto $dto): Session;
+
+    public function findByToken(string $token): ?Session;
+
+    public function deleteByToken(string $token): void;
+}