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/templates/settings/_tabs.html

@@ -3,7 +3,7 @@
 <div style="display:flex;gap:0;border-bottom:1px solid var(--border-mid);margin-bottom:1.5rem;">
   {% set tabs = [
     ("general",       "General",        "/settings/general/"),
-    ("thresholds",    "Thresholds",     "/settings/thresholds/"),
+    ("thresholds",    "Thresholds & Retention", "/settings/thresholds/"),
     ("notifications", "Notifications",  "/settings/notifications/"),
     ("reports",       "Reports",        "/settings/reports/"),
     ("auth",          "Auth",           "/settings/auth/"),