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

plugins/docker/__init__.py

@@ -21,12 +21,21 @@ def setup(app: "Quart") -> None:
     from steward.core.capabilities import register_capability
     from steward.models.users import UserRole
     from .ingest import persist_host_docker
+    from .retention import run_docker_retention
     register_capability(
         "docker.persist_host_samples", persist_host_docker,
         label="Persist host Docker samples",
         description="Store per-host container state + metrics pushed by the host agent.",
         required_role=UserRole.viewer,
     )
+    # Roll up + prune Docker time-series, driven by the core cleanup task without
+    # it importing our models. Same trusted server-side data-plane role as above.
+    register_capability(
+        "docker.run_retention", run_docker_retention,
+        label="Run Docker retention",
+        description="Roll up old docker_metrics to hourly + prune stale metrics/events.",
+        required_role=UserRole.viewer,
+    )
 
 
 def get_scheduled_tasks() -> list: