perf(metrics): a day of one metric reads from the index alone, built after boot (#5490)

#5490 "A day of one metric reads fast even cold". A cold 24 h series read
measured 18.3 s against 63 ms warm (#5485): every sample's row sat on its own
heap page. ix_plugin_metrics_series_value carries `value` (INCLUDE), so the
read is an index-only scan; it replaces the plain series index from 0023.

It is built once, with CREATE INDEX CONCURRENTLY from a task started after the
app is serving, never in a migration, so the boot isn't held while it builds
over the live table (#5220) and ingest and pages carry on. A build cut short
by a restart leaves an INVALID index; the next boot drops and rebuilds it.
Later boots find it present and only check. Postgres keeps it current on each
insert, as it did the index it replaces.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

steward/models/metrics.py

@@ -25,8 +25,12 @@ class PluginMetric(Base):
     __table_args__ = (
         Index("ix_plugin_metrics_module_resource_recorded",
               "source_module", "resource_name", "recorded_at"),
-        Index("ix_plugin_metrics_module_resource_metric_recorded",
-              "source_module", "resource_name", "metric_name", "recorded_at"),
+        # One series over a time range, read from the index alone: `value` is
+        # carried in it. Built after boot by core/metric_index.py, not by a
+        # migration, since a build over the live table takes minutes (#5490).
+        Index("ix_plugin_metrics_series_value",
+              "source_module", "resource_name", "metric_name", "recorded_at",
+              postgresql_include=["value"]),
     )