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>

steward/hosts/service.py

@@ -113,6 +113,21 @@ async def host_status(db, host_ids: list[str]) -> dict[str, dict]:
     return out
 
 
+def host_rank(status: dict | None, agent: dict | None) -> int:
+    """Where a host sorts in the Hosts list: a down monitor first, then an
+    agent gone quiet, then monitors not yet checked, then the healthy; a host
+    with neither monitors nor agent last."""
+    if status and status["state"] == "down":
+        return 0
+    if agent and not agent.get("fresh"):
+        return 1
+    if status and status["state"] == "pending":
+        return 2
+    if status or agent:
+        return 3
+    return 4
+
+
 async def host_monitors(db, host_id: str) -> list[dict]:
     """Display rows for the monitors linked to one host (host hub section)."""
     monitors = (await db.execute(
@@ -134,17 +149,6 @@ async def host_monitors(db, host_id: str) -> list[dict]:
     return data
 
 
-async def agent_metrics_by_host(db, host_names: list[str]) -> dict[str, dict]:
-    """Latest agent cpu/mem per host name, from plugin_metric_latest.
-
-    Core-safe (no host_agent import), so the Hosts list can show an
-    at-a-glance agent column without importing the host_agent plugin.
-    """
-    from steward.core.metric_series import latest_by_resource
-    latest = await latest_by_resource(db, "host_agent", host_names, ("cpu_pct", "mem_used_pct"))
-    return {name: {m: r.value for m, r in metrics.items()} for name, metrics in latest.items()}
-
-
 async def agent_overview_by_host(
     db, host_names: list[str], stale_after_seconds: int = 180,
 ) -> dict[str, dict]: