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

plugins/host_agent/routes.py

@@ -14,7 +14,7 @@ from steward.models.users import UserRole
 from sqlalchemy import select, func, or_, and_
 from datetime import timedelta
 
-from steward.core.settings import public_base_url
+from steward.core.settings import get_setting, public_base_url
 from steward.core.time_range import parse_range, RANGE_OPTIONS
 from steward.models.hosts import Host
 from steward.models.metrics import PluginMetric
@@ -637,7 +637,8 @@ async def host_detail_charts(host_id: str):
             select(Host).where(Host.id == host_id))).scalar_one_or_none()
         if host is None:
             return "", 404
-        series = await _history_for_host(session, host.name, since)
+        raw_days = int(await get_setting(session, "metrics.retention.raw_days") or 7)
+        series = await _history_for_host(session, host.name, since, raw_days=raw_days)
     return await render_template("_host_charts.html", series=series, range_key=range_key)