perf(metrics): a latest-value table for "what is it now"; dashboard cells say loading, failed and as-of (#5208)

The dashboard's host widgets took 18-60 s. Every "latest reading" query
(hosts overview, host-agent fleet, host page, vitals, SNMP, MCP get_host)
searched plugin_metrics for the newest row per metric. Postgres has no skip
scan, so each one read every raw sample in the 7-day window.

- plugin_metric_latest: one row per (source, resource, metric).
  record_metrics upserts it in the same transaction, moving forward only and
  in sorted key order. Migration 0030 seeds it by stepping the existing index.
- Retention drops a latest row once its sample leaves the raw window, so a
  resource that stopped reporting disappears when its history does, as before.
  Renames carry it along.
- A quiet host keeps its last values, marked stale. A time-bounded query
  would have shown it as missing.
- Vitals sparklines span the 6 h they claim, averaged in SQL to ~120 points,
  instead of fetching 6 h raw and drawing the last 120 samples.
- Dashboard and share-view cells: "Loading…" until the first answer; a 20 s
  deadline per fetch; "Couldn't load (why)" with Retry now and an automatic
  retry for cells that don't poll; "as of HH:MM" in the header, and
  "couldn't refresh · as of" when a refresh fails over shown data.

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

plugins/host_agent/metrics_query.py

@@ -9,7 +9,7 @@ from __future__ import annotations
 
 from datetime import datetime, timedelta, timezone
 
-from sqlalchemy import func, or_, select
+from sqlalchemy import func, select
 
 from steward.core.time_range import bucket_seconds
 from steward.models.metrics import PluginMetric, PluginMetricHourly
@@ -27,30 +27,14 @@ HISTORY_METRICS = (
 
 async def _latest_metrics_for_host(session, host_name: str) -> dict[str, dict[str, float]]:
     """{resource_name: {metric: value}} — latest sample per (resource, metric) for
-    a host + its sub-resources.
-
-    DISTINCT ON picks the newest row per group in one index-ordered pass over
-    ix_plugin_metrics_module_resource_metric_recorded, instead of a GROUP-BY-max
-    subquery self-joined back to the table (two passes over the whole history).
+    a host + its sub-resources, from plugin_metric_latest (#5208). A host whose
+    agent has gone quiet keeps its last values until they age out of the raw
+    window; the caller decides staleness from the registration's last_seen_at.
     """
-    rows = (await session.execute(
-        select(PluginMetric)
-        .where(
-            PluginMetric.source_module == SOURCE_MODULE,
-            or_(
-                PluginMetric.resource_name == host_name,
-                PluginMetric.resource_name.like(host_name + ":%"),
-            ),
-        )
-        .distinct(PluginMetric.resource_name, PluginMetric.metric_name)
-        .order_by(
-            PluginMetric.resource_name,
-            PluginMetric.metric_name,
-            PluginMetric.recorded_at.desc(),
-        )
-    )).scalars().all()
+    from steward.core.metric_series import latest_values
+
     out: dict[str, dict[str, float]] = {}
-    for r in rows:
+    for r in await latest_values(session, SOURCE_MODULE, host_name, include_children=True):
         out.setdefault(r.resource_name, {})[r.metric_name] = r.value
     return out