file-backed sessions from before a broker restart never expire #172

Found in the PR #170 review, with a failing test.

Defect: in the file backend, ClientSession.last_seen is #[serde(skip, default = SystemTime::now)] (storage/mod.rs), so every read from disk resets it to now. cleanup_expired (file_backend.rs) re-reads sessions that exist only on disk on every sweep tick, so their expiry clock restarts each time and they never expire.

Effect: a client that never reconnects after a broker restart leaks its session file, and its queue, forever.

Fix: persist the disconnect time (or last_seen) so expiry counts from the actual disconnect across restarts. A session that was connected when the broker went down should count from the restart, or from the last persisted activity.

Test: a session with 1 s expiry, reloaded from disk and swept 3 times, is still on disk.