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/migrations/env.py

@@ -15,7 +15,7 @@ sys.path.insert(0, str(Path(__file__).parent.parent.parent.parent))
 
 from steward.models.base import Base
 import steward.models  # noqa: F401
-from plugins.docker.models import DockerContainer, DockerMetric  # noqa: F401
+from plugins.docker.models import DockerContainer  # noqa: F401
 
 config = context.config
 if config.config_file_name is not None: