feat(host_agent): how far each host's clock is from Steward's (milestone 490)

- Agent 1.12.0 stamps each post with sent_at; ingest compares it with the
  moment the post arrived: clock_offset_s / clock_checked_at on the
  registration (host_agent_003), and a clock_off_secs metric (either way)
  that alert rules can watch. Replaces the old skew log, which measured the
  newest sample and was off by up to an interval.
- Settings -> Thresholds: Clock drift, 30 s warn / 5 min critical.
- Host page: 'clock 0.3s ahead' in the vitals line, toned past the warning.
- Hosts list: names hosts whose clock is off; when every reporting host is
  off by about the same amount, says Steward's own clock is the likely one.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

steward/hosts/routes.py

@@ -92,7 +92,10 @@ async def _list_page(add: dict | None = None):
 
     ctx = dict(hosts=shown, total=len(hosts), host_status=host_status, uptime=uptime, agent=agent,
                groups=groups, group_names=group_names, f=f, filter_url=filter_url, sort_url=sort_url,
-               states=host_svc.HOST_STATES, no_group=host_svc.NO_GROUP, add=add)
+               states=host_svc.HOST_STATES, no_group=host_svc.NO_GROUP, add=add,
+               clock=host_svc.clock_trouble(
+                   agent, (current_app.config.get("THRESHOLDS", {}).get("clock") or {"warn": 30})["warn"]),
+               host_ids={h.name: h.id for h in hosts})
     # The filter bar's swap gets the table alone. A history restore (Back to a
     # pushed URL) is an HTMX request too but wants the page, so key on the
     # target; Vary keeps the browser from caching one as the other.