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/formatting.py

@@ -311,6 +311,7 @@ METRICS: dict[str, tuple[str, str]] = {
     "total_clients": ("Connected clients", "devices on the network"),
     "cpu_pct": ("CPU", "CPU in use; for a container, 100% is one core"),
     "mem_pct": ("Memory (% of limit)", "memory in use, against the container's limit or the host's RAM"),
+    "mem_usage_bytes": ("Memory used", "a container's memory in use, in bytes"),
     "restart_count": ("Restarts", "times the container has restarted, in total"),
     "disk_total_bytes": ("Docker disk use", "everything Docker keeps on the host (resource: the host)"),
     "disk_images_bytes": ("Docker images", "space Docker's images take on the host"),