feat(ui): alerts, monitors, hosts, schedules and maintenance lists put problems first, with since-times (#5210)

#5210 "Problems first, with since-time, across every list" (milestone 477),
first batch.

- Alerts: the Active list reads firing, no data, acknowledged, pending, the
  longest-running first, and re-polls itself. An acknowledged alert says how
  long it has been breaching (from the alert's events; acknowledging moves
  last_transition_at) and, apart from that, who acknowledged it and when.
  Rules show what each is doing now; disabled rules sort last. The current
  value reads the latest-value table (#5208) instead of plugin_metrics.
- Monitors: a down monitor says "Down 12m · connection refused" in the list.
- Hosts: sorted down, agent gone quiet, pending, healthy. The agent column
  shows "—" for a missing reading (was 0%) and a Stale badge with its age.
  agent_metrics_by_host is gone; the list reads agent_overview_by_host.
- Schedules: failing first (failed or interrupted run, or a firing that could
  not start one), then running, then the rest, paused last; status badge
  from the shared states.
- Maintenance windows: active (ending soonest), then upcoming (starting
  soonest), with expired folded away; each says when it ends/starts/ended.

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

tests/integration/test_metric_latest.py

@@ -72,7 +72,7 @@ def test_the_newest_sample_wins_and_a_late_backlog_does_not_move_it_back(app):
 
 def test_a_quiet_host_keeps_its_last_values_and_reads_stale_not_missing(app):
     from steward.core.alerts import record_metrics
-    from steward.hosts.service import agent_metrics_by_host, agent_overview_by_host
+    from steward.hosts.service import agent_overview_by_host
 
     now = datetime.now(timezone.utc)
     quiet, live = ("quiet-" + uuid.uuid4().hex[:8]), ("live-" + uuid.uuid4().hex[:8])
@@ -87,18 +87,15 @@ def test_a_quiet_host_keeps_its_last_values_and_reads_stale_not_missing(app):
                     _sample(quiet + ":/", "disk_used_pct", 56.0, two_hours_ago),
                     _sample(live, "cpu_pct", 7.0, now - timedelta(seconds=20)),
                 ])
-            overview = await agent_overview_by_host(s, [quiet, live, "never-" + quiet], 180)
-            metrics = await agent_metrics_by_host(s, [quiet, live])
-            return overview, metrics
+            return await agent_overview_by_host(s, [quiet, live, "never-" + quiet], 180)
 
-    overview, metrics = asyncio.run(go())
+    overview = asyncio.run(go())
     q = overview[quiet]
     assert (q["cpu_pct"], q["mem_used_pct"], q["disk_root"]) == (12.0, 34.0, 56.0)
     assert q["fresh"] is False
     assert abs((q["last_at"] - two_hours_ago).total_seconds()) < 1
     assert overview[live]["fresh"] is True
     assert "never-" + quiet not in overview          # no samples at all: absent
-    assert metrics[quiet] == {"cpu_pct": 12.0, "mem_used_pct": 34.0}
 
 
 def test_retention_drops_a_latest_value_with_its_raw_history(app):