feat: host-level watch flag suppresses notifications; filter dashboard/overview by owner/manager; add ZFS monitor plugin

- watch: true (default) per host; watch: false suppresses all notifications
  for that host in udp.py and threshold.py
- Live Dashboard and Host Overview now show only hosts where the logged-in
  user is owner or manager (admins see all); WebSocket broadcasts filtered
  per-connection by the same rule
- Add hbd/client/plugins/zfs_monitor.py: collects per-pool health, capacity,
  fragmentation, dedup ratio, and cumulative I/O ops/bandwidth via zpool(8)

Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

hbd/server/settings.py

@@ -188,7 +188,7 @@ def get_settings_sections(config: dict) -> list:
             continue
         hosts_list.append({
             "name": hname,
-            "watch": bool(hcfg.get("watch", False)),
+            "watch": bool(hcfg.get("watch", True)),
             "dyndns": bool(hcfg.get("dyndns", False)),
             "owner": hcfg.get("owner", ""),
             "managers": hcfg.get("managers", []),