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/settings/routes.py

@@ -149,8 +149,6 @@ _THRESHOLD_FIELDS = [
 # Docker time-series retention windows (days). Read fresh by the cleanup task,
 # so a save takes effect on the next hourly run — no app.config wiring needed.
 _RETENTION_FIELDS = [
-    ("docker_metrics_raw_days", "docker.retention.metrics_raw_days"),
-    ("docker_metrics_rollup_days", "docker.retention.metrics_rollup_days"),
     ("docker_events_days", "docker.retention.events_days"),
     ("docker_logs_retention_days", "docker.logs.retention_days"),
     ("metrics_raw_days", "metrics.retention.raw_days"),