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/settings/routes.py

@@ -138,6 +138,7 @@ _THRESHOLD_FIELDS = [
     ("uptime_warn", "thresholds.uptime_warn", True), ("uptime_crit", "thresholds.uptime_crit", True),
     ("latency_warn", "ping.threshold.good_ms", False), ("latency_crit", "ping.threshold.warn_ms", False),
     ("tls_warn", "thresholds.tls_warn_days", False), ("tls_crit", "thresholds.tls_crit_days", False),
+    ("clock_warn", "thresholds.clock_warn_s", False), ("clock_crit", "thresholds.clock_crit_s", False),
     ("crash_loop_deaths", "thresholds.crash_loop_deaths", False),
 ]