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/__init__.py

@@ -67,5 +67,7 @@ def get_nav() -> list[dict]:
 def get_alert_metrics() -> dict[str, list[str]]:
     """What ingest.py records per container (resource "<host>/<container>"):
     restart_count is cumulative (alert on crash-looping); is_healthy is 1.0 /
-    0.0 from the container's HEALTHCHECK, only for containers that define one."""
-    return {"docker": ["cpu_pct", "mem_pct", "restart_count", "is_healthy"]}
+    0.0 from the container's HEALTHCHECK, only for containers that define one.
+    Per host (resource "<host>"), what Docker keeps on its disk (#5304)."""
+    from .disk_view import DISK_METRICS
+    return {"docker": ["cpu_pct", "mem_pct", "restart_count", "is_healthy", *DISK_METRICS]}