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/models/metrics.py

@@ -30,6 +30,25 @@ class PluginMetric(Base):
     )
 
 
+class PluginMetricLatest(Base):
+    """The newest sample of each (source, resource, metric), kept beside
+    plugin_metrics by record_metrics so "what is it now" never scans history.
+
+    Postgres has no skip scan, so a newest-per-key query over plugin_metrics
+    reads every raw sample in the window: on a seven-day window that took a
+    dashboard widget a minute (#5208). This table is one row per key. Retention
+    drops a row once its sample has aged out of the raw window, so a resource
+    that stopped reporting disappears when its raw history does.
+    """
+    __tablename__ = "plugin_metric_latest"
+
+    source_module: Mapped[str] = mapped_column(String(64), primary_key=True)
+    resource_name: Mapped[str] = mapped_column(String(255), primary_key=True)
+    metric_name: Mapped[str] = mapped_column(String(128), primary_key=True)
+    value: Mapped[float] = mapped_column(Float, nullable=False)
+    recorded_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), nullable=False)
+
+
 class PluginMetricHourly(Base):
     """Hourly rollup of plugin_metrics — the coarse series that retention rolls
     raw samples into before pruning them, so multi-day/week history stays cheap.