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>

plugins/host_agent/models.py

@@ -2,7 +2,7 @@
 from __future__ import annotations
 import uuid
 from datetime import datetime, timezone
-from sqlalchemy import Column, String, DateTime, ForeignKey
+from sqlalchemy import Column, String, DateTime, Float, ForeignKey
 from steward.models.base import Base
 
 
@@ -29,6 +29,11 @@ class HostAgentRegistration(Base):
     # Agent-reported primary IP. 45 chars = max textual IPv6 (no zone suffix).
     host_ip = Column(String(45), nullable=True)
     last_seen_at = Column(DateTime(timezone=True), nullable=True)
+    # How far this host's clock is from Steward's, in seconds, positive when
+    # the host is ahead, measured from the agent's sent_at on the newest post;
+    # and when that was. None until an agent that stamps its posts reports.
+    clock_offset_s = Column(Float, nullable=True)
+    clock_checked_at = Column(DateTime(timezone=True), nullable=True)
     created_at = Column(DateTime(timezone=True), nullable=False, default=_utcnow)
     updated_at = Column(DateTime(timezone=True), nullable=False,
                         default=_utcnow, onupdate=_utcnow)