refactor(docker): container CPU and memory live in the one metric store (#5752)

#5752 "Container CPU and memory history lives in the one metric store;
docker_metrics and docker_metrics_hourly are retired". Container cpu_pct and
mem_pct already went to plugin_metrics for alerting; now mem_usage_bytes does
too, and every snapshot in an ingest (backlog included) is recorded at its
own time. The container history chart, Busiest Containers, a service's
history and the MCP container tools read plugin_metrics under resource
"<host>/<container>", rolled up and pruned by the shared metric retention.

docker_012_drop_metrics drops both tables. Their history is not copied
(lesson #5221 "A data migration's cost is set by the live table"); the
store already holds 7 days of container CPU and memory %. Docker retention
now prunes only lifecycle events, and Settings -> Thresholds has one
"Metric retention" card for every source.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

steward/core/settings.py

@@ -117,11 +117,8 @@ DEFAULTS: dict[str, Any] = {
     # Days an ended metric anomaly waits to be reviewed before it lapses:
     # it leaves the to-review list and is kept as not reviewed (#5740).
     "anomalies.review_days": 14,
-    # 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 lifecycle events are light; keep a month (rule 25 — tunable, no
+    # restart). Container CPU and memory follow metrics.retention.* (#5752).
     "docker.retention.events_days": 30,
     # Container logs (m79): on by default for every container (operator
     # preference). `exclude` names containers the server drops on ingest; the