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/static/steward.css

@@ -371,6 +371,12 @@ body.dash-editing .widget-drawer { transform:translateY(0); }
 .widget-header { display:flex; justify-content:space-between; align-items:center; margin-bottom:0.75rem; }
 .widget-title { font-family:var(--fs-font-body); font-size:var(--fs-size-tiny); font-weight:500; color:var(--text-muted); text-transform:uppercase; letter-spacing:var(--fs-tracking-tiny); }
 .widget-link { font-size:0.8rem; color:var(--text-muted); }
+/* A widget cell's own state (#5208): loading, couldn't load, and how fresh
+   the shown data is. Set by static/dash-cells.js. */
+.widget-asof { font-size:var(--fs-size-tiny); color:var(--text-dim); white-space:nowrap; }
+.widget-asof-failed { color:var(--yellow); }
+.cell-state { font-size:0.85rem; color:var(--text-muted); padding:0.25rem 0; }
+.cell-state-failed { display:flex; flex-direction:column; align-items:flex-start; gap:0.5rem; color:var(--yellow); }
 .widget-link:hover { color:var(--text); }
 
 /* Time range selector */