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>

steward/core/formatting.py

@@ -208,6 +208,12 @@ METRICS: dict[str, tuple[str, str]] = {
     "cpu_pct": ("CPU", "CPU in use; for a container, 100% is one core"),
     "mem_pct": ("Memory (% of limit)", "memory in use, against the container's limit or the host's RAM"),
     "restart_count": ("Restarts", "times the container has restarted, in total"),
+    "disk_total_bytes": ("Docker disk use", "everything Docker keeps on the host (resource: the host)"),
+    "disk_images_bytes": ("Docker images", "space Docker's images take on the host"),
+    "disk_containers_bytes": ("Container files", "what containers wrote on top of their images"),
+    "disk_volumes_bytes": ("Docker volumes", "space Docker's volumes take on the host"),
+    "disk_build_cache_bytes": ("Docker build cache", "left by docker build; safe to clear"),
+    "disk_reclaimable_bytes": ("Reclaimable by Docker", "held by images no container uses"),
     "mem_used_pct": ("Memory used", "memory in use"),
     "mem_available_bytes": ("Memory available", "memory free for new work"),
     "swap_used_bytes": ("Swap used", "memory paged out to disk"),