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/metric_series.py

@@ -11,7 +11,7 @@ from datetime import datetime, timedelta, timezone
 from sqlalchemy import func, or_, select, text
 
 from steward.core.time_range import bucket_seconds
-from steward.models.metrics import PluginMetric, PluginMetricHourly
+from steward.models.metrics import PluginMetric, PluginMetricHourly, PluginMetricLatest
 
 # Loose index scans: step through the distinct leading values of the
 # (source_module, resource_name, …) index one probe at a time instead of
@@ -48,26 +48,49 @@ async def resources(db, source_module: str) -> list[str]:
     return [v for (v,) in await db.execute(_RESOURCES_SQL, {"src": source_module})]
 
 
+def _resource_match(resource: str, include_children: bool):
+    match = PluginMetricLatest.resource_name == resource
+    if include_children:
+        like = resource.replace("\\", "\\\\").replace("%", "\\%").replace("_", "\\_") + ":%"
+        match = or_(match, PluginMetricLatest.resource_name.like(like, escape="\\"))
+    return match
+
+
 async def latest_values(db, source_module: str, resource: str, *,
-                        include_children: bool = False) -> list[PluginMetric]:
+                        include_children: bool = False) -> list[PluginMetricLatest]:
     """The newest sample of each metric for a resource, sorted by (resource, metric).
 
     include_children also returns the resource's sub-resources, named
-    "<resource>:<part>" (a host's mounts, interfaces, sensors).
+    "<resource>:<part>" (a host's mounts, interfaces, sensors). Read from
+    plugin_metric_latest, so the cost is the resource's metric count, not its
+    history.
     """
-    match = PluginMetric.resource_name == resource
-    if include_children:
-        like = resource.replace("\\", "\\\\").replace("%", "\\%").replace("_", "\\_") + ":%"
-        match = or_(match, PluginMetric.resource_name.like(like, escape="\\"))
     return list((await db.execute(
-        select(PluginMetric)
-        .where(PluginMetric.source_module == source_module, match)
-        .distinct(PluginMetric.resource_name, PluginMetric.metric_name)
-        .order_by(PluginMetric.resource_name, PluginMetric.metric_name,
-                  PluginMetric.recorded_at.desc())
+        select(PluginMetricLatest)
+        .where(PluginMetricLatest.source_module == source_module,
+               _resource_match(resource, include_children))
+        .order_by(PluginMetricLatest.resource_name, PluginMetricLatest.metric_name)
     )).scalars())
 
 
+async def latest_by_resource(db, source_module: str, resources, metrics=None,
+                             ) -> dict[str, dict[str, PluginMetricLatest]]:
+    """{resource: {metric: newest sample}} for several resources of a source,
+    optionally only the named metrics. A resource with no samples is absent."""
+    resources = list(resources)
+    if not resources:
+        return {}
+    q = select(PluginMetricLatest).where(
+        PluginMetricLatest.source_module == source_module,
+        PluginMetricLatest.resource_name.in_(resources))
+    if metrics is not None:
+        q = q.where(PluginMetricLatest.metric_name.in_(list(metrics)))
+    out: dict[str, dict[str, PluginMetricLatest]] = {}
+    for r in (await db.execute(q)).scalars():
+        out.setdefault(r.resource_name, {})[r.metric_name] = r
+    return out
+
+
 async def points(db, source_module: str, resource: str, metric: str, since: datetime,
                  *, target: int = 120) -> list[tuple[datetime, float]]:
     """(time, average) points for one metric since `since`, oldest first.