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

tests/core/test_metrics_retention.py

@@ -0,0 +1,17 @@
+"""Unit tests for the plugin_metrics rollup cutoff helpers (no DB)."""
+from datetime import datetime, timezone
+
+from steward.core.metrics_retention import _hour_floor, _rollup_cutoff
+
+
+def test_hour_floor_drops_sub_hour():
+    dt = datetime(2026, 6, 20, 15, 42, 9, 123456, tzinfo=timezone.utc)
+    assert _hour_floor(dt) == datetime(2026, 6, 20, 15, 0, 0, 0, tzinfo=timezone.utc)
+
+
+def test_rollup_cutoff_is_hour_aligned_and_offset():
+    now = datetime(2026, 6, 20, 15, 42, 9, tzinfo=timezone.utc)
+    cutoff = _rollup_cutoff(now, 7)
+    assert (cutoff.minute, cutoff.second, cutoff.microsecond) == (0, 0, 0)
+    # 7 whole days back, then floored to the hour.
+    assert cutoff == datetime(2026, 6, 13, 15, 0, 0, 0, tzinfo=timezone.utc)