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/metrics_retention.py

@@ -49,7 +49,7 @@ async def rollup_plugin_metrics(
     from sqlalchemy import delete, func, select
     from sqlalchemy.dialects.postgresql import insert as pg_insert
 
-    from steward.models.metrics import PluginMetric, PluginMetricHourly
+    from steward.models.metrics import PluginMetric, PluginMetricHourly, PluginMetricLatest
 
     if now is None:
         now = datetime.now(timezone.utc)
@@ -97,6 +97,11 @@ async def rollup_plugin_metrics(
         await session.execute(
             delete(PluginMetric).where(PluginMetric.recorded_at < raw_cutoff)
         )
+    # A latest value goes with its raw history: a resource that stopped
+    # reporting a raw window ago is no longer "now" anywhere.
+    latest_pruned = (await session.execute(
+        delete(PluginMetricLatest).where(PluginMetricLatest.recorded_at < raw_cutoff)
+    )).rowcount or 0
 
     # ── 2. Prune rolled-up rows beyond the rollup window ──
     rollup_cutoff = now - timedelta(days=rollup_days)
@@ -109,4 +114,5 @@ async def rollup_plugin_metrics(
         "buckets_rolled": rolled,
         "raw_rows_rolled": rolled_rows,
         "rollup_pruned": rollup_pruned,
+        "latest_pruned": latest_pruned,
     }