feat(metrics): roll plugin_metrics up to hourly to bound storage

plugin_metrics grows by (sources × resources × ~30s cadence); keeping 90d of raw
is a large table. Add a raw→hourly rollup (mirroring the Docker plugin) so only a
short raw window is kept at full resolution, with hourly averages archived longer.

- PluginMetricHourly model + core migration 0024 (plugin_metrics_hourly: avg/max/
  count per source/resource/metric/hour, unique bucket constraint + lookup index).
- steward/core/metrics_retention.rollup_plugin_metrics: date_trunc('hour') agg of
  raw older than the hour-aligned raw window, idempotent pg upsert into hourly,
  delete the rolled raw, prune hourly beyond the rollup window.
- cleanup.py: plugin_metrics is no longer blanket-deleted at data.retention_days;
  _run_metrics_retention drives the rollup with windows read live from settings.
- Settings: metrics.retention.raw_days (7) + rollup_days (90), tunable on the
  Thresholds & Retention page (new "Host metrics retention" card).
- Chart read: _history_for_host merges the hourly rollup (older part of the range)
  with raw date_bin (recent part, capped ≤1h), so 30d charts keep working —
  recent at full resolution, older at hourly. Route passes raw_days from settings.
- Tests: unit (cutoff helpers) + integration (rollup aggregates/prunes; history
  merges hourly + raw) against Postgres.

Speed was already handled by the indexes + SQL aggregation; this is the storage
lever (raw window ~10x smaller).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016Jg27rgypiW2efULXJDtMC

steward/templates/settings/thresholds.html

@@ -70,6 +70,21 @@
           "Keep container start/stop/die/health history this long.") }}
 </div>
 
+<div class="card" style="max-width:640px;margin-top:1rem;">
+  <h2 class="section-title" style="margin-bottom:0.5rem;">Host metrics retention</h2>
+  <p style="font-size:0.82rem;color:var(--text-muted);margin-bottom:1.25rem;">
+    Bounds how much host-agent metric history is stored (CPU, memory, disk, network,
+    temps, …). Raw per-sample points are kept for the raw window, then rolled up
+    into hourly averages kept for the rollup window. Host charts read raw for the
+    recent part of a range and hourly for older data. Applied by the hourly cleanup.
+  </p>
+
+  {{ days("Raw metrics", "metrics_raw_days", "metrics.retention.raw_days",
+          "Keep per-sample host metrics this long, then roll up to hourly averages.") }}
+  {{ days("Rolled-up metrics", "metrics_rollup_days", "metrics.retention.rollup_days",
+          "Keep the hourly-averaged series this long for multi-week history.") }}
+</div>
+
 <div style="margin-top:1rem;display:flex;align-items:center;gap:1rem;">
   <button type="submit" class="btn">Save</button>
   <span style="font-size:0.82rem;color:var(--text-muted);">Takes effect immediately — no restart.</span>