feat(docker): retention + hourly rollup for metrics/events with Settings windows

Bounds Docker time-series growth (the main scaling concern). New
docker_metrics_hourly table + docker_006 migration; a plugin retention module
(docker.run_retention capability) rolls raw docker_metrics older than the raw
window into hourly averages (idempotent upsert), deletes the rolled raw rows,
then prunes stale rollups + lifecycle events. Core cleanup.py drives it each
hourly run via the capability (no plugin-model import), reading the three
retention windows fresh from settings so changes apply without restart (rule 25).

Settings → "Thresholds & Retention" gains a Docker retention card (raw /
rolled-up / events windows, working defaults 7/90/30 days). Unit tests cover the
hour-aligned cutoff/bucketing helpers; integration test exercises the real
rollup-average + prune across both windows.

Milestone 77 task #941.

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

steward/core/settings.py

@@ -78,6 +78,12 @@ DEFAULTS: dict[str, Any] = {
     "thresholds.load_warn": 80, "thresholds.load_crit": 100,
     "thresholds.temp_warn": 70, "thresholds.temp_crit": 85,
     "thresholds.uptime_warn": 99.0, "thresholds.uptime_crit": 95.0,
+    # Docker time-series retention (rule 25 — tunable, no restart). Raw 30s
+    # samples are heavy, so keep a short raw window then roll up to hourly
+    # averages kept much longer; lifecycle events are light, keep a month.
+    "docker.retention.metrics_raw_days": 7,
+    "docker.retention.metrics_rollup_days": 90,
+    "docker.retention.events_days": 30,
     "plugins.index_url": "https://git.fabledsword.com/bvandeusen/Steward-plugins/raw/branch/main/index.yaml",
     # Default-enabled plugins. These are the generic, non-vendor-specific
     # bundled plugins (protocols/standards, not a single product) — useful on