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>

steward/core/renames.py

@@ -11,7 +11,7 @@ from sqlalchemy import and_, delete, func, or_, update
 from sqlalchemy.ext.asyncio import AsyncSession
 
 from steward.models.alerts import AlertRule, MaintenanceWindow
-from steward.models.metrics import PluginMetric, PluginMetricHourly
+from steward.models.metrics import PluginMetric, PluginMetricHourly, PluginMetricLatest
 
 # What a host's name keys, and how each source names a part of the host:
 # host_agent "web01:/var", "web01:core3"; docker "web01/nginx".
@@ -47,7 +47,7 @@ async def carry_rename(session: AsyncSession, sources, old: str, new: str) -> di
     if not old or old == new:
         return {}
     sources = tuple(sources)
-    for model in (PluginMetric, PluginMetricHourly):
+    for model in (PluginMetric, PluginMetricHourly, PluginMetricLatest):
         await session.execute(delete(model).where(
             _matches(model.source_module, model.resource_name, sources, new)))
     moved = {}
@@ -58,6 +58,8 @@ async def carry_rename(session: AsyncSession, sources, old: str, new: str) -> di
         ("metric_rows", PluginMetric, PluginMetric.source_module, PluginMetric.resource_name),
         ("hourly_rows", PluginMetricHourly, PluginMetricHourly.source_module,
          PluginMetricHourly.resource_name),
+        ("latest_rows", PluginMetricLatest, PluginMetricLatest.source_module,
+         PluginMetricLatest.resource_name),
     ):
         result = await session.execute(
             update(model).where(_matches(src, res, sources, old))