Three edge cases broke annotations on reboot or interrupted sessions:
- state.pop() ran before grafana_close(), so a failed PATCH (Grafana
still restarting after reboot) permanently lost the grafana_id and
left the annotation open forever in Grafana.
- a single poll with no sessions closed every active annotation, so
Jellyfin restarts or client reconnects produced spurious close +
duplicate-open pairs.
- timeEnd was always now_ms, so a reboot during playback wrote an
annotation reading as if the user watched through the outage.
Fix: track last_seen_ms and missing_count in state; retain entries
until grafana_close succeeds (retry indefinitely); require
MISSING_THRESHOLD absent polls before close; clamp close_time to
last_seen_ms + (MISSING_THRESHOLD + 1) * POLL_INTERVAL.
Adds three subtests in tests/jellyfin-annotations.nix that each fail
on the old code and pass on the new.
