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>

docs/reference/code-map.md

@@ -89,6 +89,7 @@ Quick reference for where key functions, models, and entry points live in the co
 | `AlertState` | `steward/models/alerts.py` | `alert_states` |
 | `AlertEvent` | `steward/models/alerts.py` | `alert_events` |
 | `PluginMetric` | `steward/models/metrics.py` | `plugin_metrics` |
+| `PluginMetricLatest` | `steward/models/metrics.py` | `plugin_metric_latest` |
 | `AnsibleRun` | `steward/models/ansible.py` | `ansible_runs` |
 | `User` | `steward/models/users.py` | `users` |
 | `AppSetting` | `steward/models/settings.py` | `app_settings` |