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_monitors_page.py

@@ -1,5 +1,6 @@
 """The Monitors page leads with what needs looking at (#5121): a count strip,
 problems first, each monitor on a page of its own, adding behind a button."""
+import re
 from datetime import datetime, timedelta, timezone
 from types import SimpleNamespace
 
@@ -115,3 +116,24 @@ async def test_adding_is_its_own_page_and_keeps_what_was_typed(app):
     assert '<a href="/monitors/">Monitors</a>' in page and 'action="/monitors/add"' in page
     assert '<p class="field-error" role="alert">port must be between 1 and 65535, got 0.</p>' in page
     assert 'value="db"' in page and 'value="10.0.0.9"' in page and 'value="30"' in page
+
+
+def test_down_reason_says_why_in_a_few_words():
+    from steward.monitors.service import down_reason
+    assert down_reason(_check(False)) == "503 Service Unavailable"       # the probe's own error first
+    assert down_reason(SimpleNamespace(error_msg=None, status_code=502, content_matched=None)) == "HTTP 502"
+    assert down_reason(SimpleNamespace(error_msg=None, status_code=None, content_matched=False)) == \
+        "expected content not found"
+    assert down_reason(None) is None
+
+
+async def test_a_down_monitor_says_since_when_and_why_in_the_list(app):
+    row = _row(_mon("api", mid="m2"), "down", _check(False), 90.0)
+    row.update(down_since=datetime.now(timezone.utc) - timedelta(minutes=12),
+               down_reason="connection refused")
+    async with app.test_request_context("/monitors/rows"):
+        page = await render_template("monitors/rows.html", monitor_data=[row],
+                                     counts={"up": 0, "degraded": 0, "down": 1, "pending": 0, "paused": 0},
+                                     range_key="24h")
+    page = " ".join(page.split())
+    assert re.search(r'Down <time [^>]*>12m</time> · connection refused', page), page