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/backup/contents.py

@@ -40,7 +40,7 @@ EXCLUDED: frozenset[str] = frozenset({
     # Run history; its logs live on disk, not in the database.
     "ansible_runs",
     "docker_containers", "docker_disk_usage", "docker_events", "docker_host_status",
-    "docker_images", "docker_logs", "docker_metrics", "docker_metrics_hourly",
+    "docker_images", "docker_logs",
     "docker_swarm_nodes", "docker_swarm_services",
     "traefik_access_summaries", "traefik_certs", "traefik_global_stats", "traefik_metrics",
     "traefik_status_codes",