feat(timeline): maintenance, reboots, deploys, crash-loops and reviewed anomalies on the timeline (#6349)

The host agent writes each maintenance job once it ends, and each stretch a
host was away (a reboot, a crash, lost contact, the agent restarting), kept up
to date until it is back. Docker judges each snapshot: a unit on a new image,
or a container recreated, is a deploy; deaths past the Thresholds crash-loop
count are one crash-loop entry that later deaths extend; single restarts are
recorded but hidden. An anomaly has one entry whose type follows its review:
only those judged real are shown by default.

A source whose record changes keeps one entry for it (record_or_update).
Plugins register the records their entries point to, so an aged-out job or
downtime reads "Details no longer kept". The first fill runs once per source
(timeline.filled), the host agent's jobs and downtime included.

get_anomaly's "what else was going on" reads the timeline for the host or
resource instead of querying alerts, runs, maintenance, downtime and container
deaths itself; Docker's own source keeps only starts, stops and health
changes. A new kind, Observed, is for what Steward saw happen with no rule or
person behind it.

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

plugins/host_agent/cadence.py

@@ -22,7 +22,7 @@ from __future__ import annotations
 import json
 from datetime import datetime, time, timedelta, timezone
 
-from . import maintenance
+from . import maintenance, timeline
 
 WEEKDAYS = ("Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday", "Sunday")
 # How long a host may take to be back in service after its job finished.
@@ -241,6 +241,8 @@ async def advance(session, run, plan, now: datetime) -> str:
         if move == "fail":
             if job is not None and job.state == "done":
                 maintenance._finish(job, "failed", now, why)     # the host never came back into service
+                await timeline.write(session, f"maintenance job {job.id}",
+                                     timeline.job_entry(session, job, host.name if host else None))
             name = host.name if host else host_id
             _end(run, "failed", now, f"{name}: {why}")
         elif move == "stop":
@@ -325,5 +327,7 @@ async def failed_hosts(session, now: datetime) -> dict[str, float]:
                 HostAgentRegistration.host_id == host.id))).scalar_one_or_none()
             if await _back(session, host, job, reg, now) is None:
                 job.came_back_at = now
+                await timeline.write(session, f"maintenance job {job.id}",
+                                     timeline.job_entry(session, job, host.name))
         out[name] = 1.0 if maintenance.still_failed(job) else 0.0
     return out