feat: throttle ZFS collector when no UI is connected (#126)

[gnacho]

Closes #126.

What

The periodic ZFS collector was the main source of sudo/journald log volume on always-on nodes. This PR makes the collector aware of whether the web UI is connected via SSE.

How

• Hub.SubscriberCount() exposes the number of open SSE streams.
• ZpoolCollector receives the active/alert/idle intervals and the subscriber callback.
• With the UI open: full collection every EASYZFS_ZPOOL_INTERVAL (default 10 s).
• With the UI closed: cheap health/alert heartbeat every EASYZFS_ZPOOL_ALERT_INTERVAL (default 60 s), full collection only every EASYZFS_ZPOOL_IDLE_INTERVAL (default 300 s).
• lightCollect runs only zpool list + zpool status --json per pool; fullCollect keeps datasets, snapshots, history and series.
• Alerts still evaluate at least every minute; capacity series still persist at least every 10 minutes.

Validation

• go test ./... passes.
• Deployed a validation build on citadel-02 (192.168.1.101); service is healthy and journalctl shows 0 new easyzfs sudo log entries during a 5-minute idle window.

Env changes

• EASYZFS_ZPOOL_INTERVAL now means seconds with UI open (default 10).
• New: EASYZFS_ZPOOL_ALERT_INTERVAL (default 60) and EASYZFS_ZPOOL_IDLE_INTERVAL (default 300).