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>

tests/integration/test_host_metrics.py

@@ -1,8 +1,9 @@
-"""Integration: host-metrics read paths (DISTINCT ON latest + SQL date_bin history).
+"""Integration: host-metrics read paths (latest values + SQL date_bin history).
 
 Validates the two host_agent query helpers that back the slow host views, against
-a live Postgres: the latest-per-(resource,metric) lookup and the bucket-averaged
-history (which now aggregates in SQL instead of shipping raw rows to Python).
+a live Postgres: the latest-per-(resource,metric) lookup (plugin_metric_latest,
+kept by record_metrics) and the bucket-averaged history (which aggregates in SQL
+instead of shipping raw rows to Python).
 Requires STEWARD_DATABASE_URL.
 """
 from __future__ import annotations
@@ -31,8 +32,9 @@ def app():
 
 
 @_NEEDS_DB
-def test_latest_distinct_on_and_sql_bucketed_history(app):
+def test_latest_values_and_sql_bucketed_history(app):
     from sqlalchemy import text
+    from steward.core.alerts import record_metrics
     from steward.models.metrics import PluginMetric
     from plugins.host_agent.metrics_query import (
         SOURCE_MODULE, _history_for_host, _latest_metrics_for_host,
@@ -62,7 +64,7 @@ def test_latest_distinct_on_and_sql_bucketed_history(app):
                     metric_name="disk_used_pct", value=80.0,
                     recorded_at=now - timedelta(minutes=5),
                 ))
-                s.add_all(rows)
+                await record_metrics(s, rows)
 
             latest = await _latest_metrics_for_host(s, hostname)
             hist = await _history_for_host(s, hostname, now - timedelta(hours=1))
@@ -70,7 +72,7 @@ def test_latest_distinct_on_and_sql_bucketed_history(app):
 
     latest, hist = asyncio.run(_go())
 
-    # DISTINCT ON returns the newest sample per (resource, metric).
+    # The newest sample per (resource, metric).
     assert latest[hostname]["cpu_pct"] == 30.0
     assert latest[hostname + ":/"]["disk_used_pct"] == 80.0