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>

plugins/snmp/routes.py

@@ -3,10 +3,11 @@ from __future__ import annotations
 from datetime import datetime, timedelta, timezone
 
 from quart import Blueprint, current_app, render_template, request
-from sqlalchemy import func, select
+from sqlalchemy import select
 
 from steward.auth.middleware import require_role
 from steward.models.users import UserRole
+from steward.core.metric_series import latest_by_resource
 from steward.models.metrics import PluginMetric
 
 snmp_bp = Blueprint("snmp", __name__, template_folder="templates")
@@ -14,35 +15,8 @@ snmp_bp = Blueprint("snmp", __name__, template_folder="templates")
 
 async def _latest_readings(db, device_names: list[str]) -> dict[str, dict[str, float]]:
     """Return {device_name: {label: latest_value}} for all configured devices."""
-    if not device_names:
-        return {}
-
-    # Latest value per (resource_name, metric_name) pair
-    subq = (
-        select(
-            PluginMetric.resource_name,
-            PluginMetric.metric_name,
-            func.max(PluginMetric.recorded_at).label("latest_at"),
-        )
-        .where(PluginMetric.source_module == "snmp")
-        .where(PluginMetric.resource_name.in_(device_names))
-        .group_by(PluginMetric.resource_name, PluginMetric.metric_name)
-        .subquery()
-    )
-    result = await db.execute(
-        select(PluginMetric).join(
-            subq,
-            (PluginMetric.resource_name == subq.c.resource_name)
-            & (PluginMetric.metric_name == subq.c.metric_name)
-            & (PluginMetric.recorded_at == subq.c.latest_at),
-        ).where(PluginMetric.source_module == "snmp")
-    )
-    rows = result.scalars().all()
-
-    out: dict[str, dict[str, float]] = {}
-    for row in rows:
-        out.setdefault(row.resource_name, {})[row.metric_name] = row.value
-    return out
+    latest = await latest_by_resource(db, "snmp", device_names)
+    return {name: {m: r.value for m, r in metrics.items()} for name, metrics in latest.items()}
 
 
 async def _history(db, device_name: str, hours: int = 24) -> dict[str, list]: