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_mcp_tools.py

@@ -44,6 +44,7 @@ def test_diagnostic_tools_over_a_seeded_fleet(app):
     )
     from steward.models.ansible import AnsibleRun, AnsibleRunStatus
     from steward.models.hosts import Host
+    from steward.core.alerts import _keep_latest
     from steward.models.metrics import PluginMetric
     from steward.models.monitors import Monitor, MonitorResult
     from steward.models.users import User, UserRole
@@ -77,16 +78,21 @@ def test_diagnostic_tools_over_a_seeded_fleet(app):
                 s.add(MonitorResult(monitor_id=web_id, checked_at=now - timedelta(minutes=1),
                                     is_up=True, status_code=200, response_ms=40.0,
                                     tls_expires_at=now + timedelta(days=5)))
-                for i, v in enumerate((10.0, 20.0, 95.0)):
-                    s.add(PluginMetric(source_module="host_agent", resource_name=host_name,
-                                       metric_name="cpu_pct", value=v,
-                                       recorded_at=now - timedelta(minutes=40 - i * 20, seconds=30)))
-                s.add(PluginMetric(source_module="host_agent", resource_name=host_name,
-                                   metric_name="mem_used_pct", value=50.0,
-                                   recorded_at=now - timedelta(seconds=30)))
-                s.add(PluginMetric(source_module="host_agent", resource_name=host_name + ":/",
-                                   metric_name="disk_used_pct", value=41.0,
-                                   recorded_at=now - timedelta(seconds=30)))
+                samples = [PluginMetric(source_module="host_agent", resource_name=host_name,
+                                        metric_name="cpu_pct", value=v,
+                                        recorded_at=now - timedelta(minutes=40 - i * 20, seconds=30))
+                           for i, v in enumerate((10.0, 20.0, 95.0))]
+                samples.append(PluginMetric(source_module="host_agent", resource_name=host_name,
+                                            metric_name="mem_used_pct", value=50.0,
+                                            recorded_at=now - timedelta(seconds=30)))
+                samples.append(PluginMetric(source_module="host_agent",
+                                            resource_name=host_name + ":/",
+                                            metric_name="disk_used_pct", value=41.0,
+                                            recorded_at=now - timedelta(seconds=30)))
+                s.add_all(samples)
+                # What record_metrics keeps beside the rows, without evaluating
+                # the rule this seed sets up by hand.
+                await _keep_latest(s, samples)
                 s.add(AlertRule(id=rule_id, name=f"cpu hot {tag}", source_module="host_agent",
                                 resource_name=host_name, metric_name="cpu_pct",
                                 operator=AlertOperator.gt, threshold=90.0,