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/__init__.py

@@ -11,7 +11,7 @@ _app: "Quart | None" = None
 def setup(app: "Quart") -> None:
     global _app
     _app = app
-    from .models import DockerContainer, DockerMetric  # noqa: F401 — register with Base
+    from .models import DockerContainer  # noqa: F401 — register with Base
 
     # Publish the per-host persist hook so the host agent can hand off the
     # `docker` array from each sample without importing our models (opportunistic
@@ -28,12 +28,13 @@ def setup(app: "Quart") -> None:
         description="Store per-host container state + metrics pushed by the host agent.",
         required_role=UserRole.viewer,
     )
-    # Roll up + prune Docker time-series, driven by the core cleanup task without
-    # it importing our models. Same trusted server-side data-plane role as above.
+    # Prune lifecycle events, driven by the core cleanup task without it
+    # importing our models. Same trusted server-side data-plane role as above.
+    # Container CPU and memory live in the one metric store and its retention.
     register_capability(
         "docker.run_retention", run_docker_retention,
         label="Run Docker retention",
-        description="Roll up old docker_metrics to hourly + prune stale metrics/events.",
+        description="Prune Docker lifecycle events past their window.",
         required_role=UserRole.viewer,
     )
     register_capability(