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>

plugins/docker/routes.py

@@ -26,7 +26,7 @@ from .overview import (
     stack_of,
 )
 from .models import (
-    DockerContainer, DockerDiskUsage, DockerHostStatus, DockerImage, DockerMetric,
+    DockerContainer, DockerDiskUsage, DockerHostStatus, DockerImage,
     DockerSwarmNode, DockerSwarmService,
 )
 
@@ -303,11 +303,7 @@ async def widget_resources():
         # fill the list with duplicates of the same container.
         running = dedup_by_container_id([
             c for c in await docker_svc.list_containers(db) if c.status == "running"])
-        average = {(h, n): v for h, n, v in (await db.execute(
-            select(DockerMetric.host_id, DockerMetric.container_name, func.avg(DockerMetric.cpu_pct))
-            .where(DockerMetric.scraped_at >= since)
-            .group_by(DockerMetric.host_id, DockerMetric.container_name)
-        )).all()}
+        average = await docker_svc.average_cpu(db, {(c.host_id, c.name) for c in running}, since)
 
         def cpu_of(c):
             v = average.get((c.host_id, c.name))