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/templates/settings/thresholds.html

@@ -104,18 +104,13 @@
 </div>
 
 <div class="card" style="max-width:640px;margin-top:1.5rem;">
-  <h2 class="section-title" style="margin-bottom:0.5rem;">Docker data retention</h2>
+  <h2 class="section-title" style="margin-bottom:0.5rem;">Docker lifecycle events</h2>
   <p style="font-size:0.82rem;color:var(--text-muted);margin-bottom:1.25rem;">
-    Bounds how much Docker history is stored. Raw per-sample container metrics are
-    kept for the raw window, then rolled up into hourly averages kept for the
-    rollup window; lifecycle events are kept for the events window. Applied by the
-    hourly cleanup task — a change takes effect on its next run.
+    How long container start, stop, die and health changes are kept. Container CPU
+    and memory are kept with the other metrics, under Metric retention below.
+    Applied by the hourly cleanup task — a change takes effect on its next run.
   </p>
 
-  {{ days("Raw metrics", "docker_metrics_raw_days", "docker.retention.metrics_raw_days",
-          "Keep per-sample container CPU/memory points this long, then roll up to hourly.") }}
-  {{ days("Rolled-up metrics", "docker_metrics_rollup_days", "docker.retention.metrics_rollup_days",
-          "Keep the hourly-averaged series this long for multi-day history.") }}
   {{ days("Lifecycle events", "docker_events_days", "docker.retention.events_days",
           "Keep container start/stop/die/health history this long.") }}
 </div>
@@ -169,16 +164,18 @@
 </div>
 
 <div class="card" style="max-width:640px;margin-top:1rem;">
-  <h2 class="section-title" style="margin-bottom:0.5rem;">Host metrics retention</h2>
+  <h2 class="section-title" style="margin-bottom:0.5rem;">Metric retention</h2>
   <p style="font-size:0.82rem;color:var(--text-muted);margin-bottom:1.25rem;">
-    Bounds how much host-agent metric history is stored (CPU, memory, disk, network,
-    temps, …). Raw per-sample points are kept for the raw window, then rolled up
-    into hourly averages kept for the rollup window. Host charts read raw for the
-    recent part of a range and hourly for older data. Applied by the hourly cleanup.
+    Bounds how much metric history is stored: hosts (CPU, memory, disk, network,
+    temps, …), containers (CPU and memory), monitors and the other sources. Raw
+    per-sample points are kept for the raw window, then rolled up into hourly
+    figures (average, peak, and the level held) kept for the rollup window. Charts
+    read raw for the recent part of a range and hourly for older data. Applied by
+    the hourly cleanup.
   </p>
 
   {{ days("Raw metrics", "metrics_raw_days", "metrics.retention.raw_days",
-          "Keep per-sample host metrics this long, then roll up to hourly averages.") }}
+          "Keep per-sample metrics this long, then roll up to hourly.") }}
   {{ days("Rolled-up metrics", "metrics_rollup_days", "metrics.retention.rollup_days",
           "Keep the hourly-averaged series this long for multi-week history.") }}
 </div>