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>

tests/integration/test_mcp_plugin_tools.py

@@ -132,12 +132,16 @@ def test_docker_and_agent_tools_over_seeded_rows(app):
                         "VALUES (:id, :h, :n, :ts, :st, :line)"),
                         {"id": str(uuid.uuid4()), "h": hid, "n": web,
                          "ts": now - timedelta(minutes=5 - i), "st": stream, "line": line})
+                # Container series live in the one metric store (#5752).
                 for i, cpu in enumerate((10.0, 30.0, 50.0)):
-                    await s.execute(text(
-                        "INSERT INTO docker_metrics (id, host_id, container_name, scraped_at, "
-                        "cpu_pct, mem_pct, mem_usage_bytes) VALUES (:id, :h, :n, :t, :c, 20.0, 1000)"),
-                        {"id": str(uuid.uuid4()), "h": hid, "n": web,
-                         "t": now - timedelta(minutes=30 - i * 10), "c": cpu})
+                    for metric, value in (("cpu_pct", cpu), ("mem_pct", 20.0),
+                                          ("mem_usage_bytes", 1000.0)):
+                        await s.execute(text(
+                            "INSERT INTO plugin_metrics (id, source_module, resource_name, "
+                            "metric_name, value, recorded_at) "
+                            "VALUES (:id, 'docker', :r, :m, :v, :t)"),
+                            {"id": str(uuid.uuid4()), "r": f"{host_name}/{web}", "m": metric,
+                             "v": value, "t": now - timedelta(minutes=30 - i * 10)})
                 await s.execute(text(
                     "INSERT INTO docker_swarm_services (host_id, service_name, mode, desired, running, "
                     "placement_json, tasks_json, updated_at) "