feat(docker): Docker disk is recorded over time per host, the Disk page shows each host's growth and folds images under their repository, and the overview has one disk line per host (#5304–#5306)

- #5304 "Docker disk totals recorded over time per host, with a growth chart and alertable metrics"
- #5305 "Disk page: images grouped by repository, versions folded under it"
- #5306 "Docker overview: one disk line per host linking to the Disk page"

Each `docker system df` report is now also recorded as metrics, with the host
as the resource: disk_total_bytes, its parts, and disk_reclaimable_bytes. An
alert rule can watch them. The Disk page starts with the hosts side by side,
showing how much each grew over the selected range with a line of it, then
gives each host its own section. An image's versions fold under its
repository, which is charged only for the layers its versions don't share.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

plugins/docker/ingest.py

@@ -240,6 +240,14 @@ async def _persist_disk(session, host, disk: dict) -> None:
     summary.containers_count = int(disk.get("containers_count") or 0)
     summary.volumes_count = int(disk.get("volumes_count") or 0)
     summary.updated_at = now
+    # The totals as metrics too (#5304): a history of each host's growth, and
+    # something an alert rule can watch. Resource: the host's name.
+    from steward.core.alerts import MetricBatch
+    from .disk_view import disk_metrics
+    batch = MetricBatch()
+    for metric, value in disk_metrics(disk).items():
+        batch.add(source_module="docker", resource_name=host.name, metric_name=metric, value=value)
+    await batch.record(session)
 
     images = disk.get("images") or []
     seen: set[str] = set()