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/core/test_problems_first.py

@@ -0,0 +1,31 @@
+"""Lists put what needs looking at first (#5210): hosts by their worst
+signal, schedules failing first, maintenance windows active then upcoming."""
+from types import SimpleNamespace as NS
+
+from steward.ansible.routes import _schedule_order
+from steward.hosts.service import host_rank
+
+
+def test_hosts_sort_down_then_quiet_agent_then_pending_then_healthy():
+    down, up, pending = {"state": "down"}, {"state": "up"}, {"state": "pending"}
+    fresh, quiet = {"fresh": True}, {"fresh": False}
+    assert host_rank(down, fresh) == 0
+    assert host_rank(up, quiet) == 1            # monitors fine, but the agent went silent
+    assert host_rank(None, quiet) == 1
+    assert host_rank(pending, fresh) == 2
+    assert host_rank(up, fresh) == 3 and host_rank(None, fresh) == 3
+    assert host_rank(None, None) == 4           # nothing watches it
+
+
+def _sched(name, *, enabled=True, last_error=None, status=None):
+    run = NS(status=NS(value=status)) if status else None
+    return {"s": NS(name=name, enabled=enabled, last_error=last_error), "last_run": run}
+
+
+def test_schedules_sort_failing_first_and_paused_last():
+    rows = [_sched("nightly", status="success"), _sched("paused", enabled=False, status="failed"),
+            _sched("backup", status="failed"), _sched("deploy", status="running"),
+            _sched("broken", last_error="source not found"), _sched("cut", status="interrupted"),
+            _sched("busy", last_error="the last run is still going", status="success")]
+    assert [r["s"].name for r in sorted(rows, key=_schedule_order)] == [
+        "backup", "broken", "cut", "deploy", "busy", "nightly", "paused"]