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>

tests/integration/test_docker.py

@@ -611,7 +611,8 @@ def test_disk_usage_persisted(app):
             async with s.begin():
                 await s.execute(text("DELETE FROM docker_images"))
                 await s.execute(text("DELETE FROM docker_disk_usage"))
-                h = Host(id=str(uuid.uuid4()), name="dsk", address="10.6.6.6")
+                hid = str(uuid.uuid4())
+                h = Host(id=hid, name=f"dsk-{hid[:8]}", address="10.6.6.6")
                 s.add(h)
                 await s.flush()
                 hid = h.id
@@ -631,9 +632,17 @@ def test_disk_usage_persisted(app):
             reclaimable2 = (await s.execute(text(
                 "SELECT images_reclaimable FROM docker_disk_usage WHERE host_id=:h"),
                 {"h": hid})).scalar()
-            return summary, img_count, [r[0] for r in after], reclaimable2
+            # Each report is recorded as metrics too, resource = the host (#5304).
+            metrics = (await s.execute(text(
+                "SELECT metric_name, value FROM plugin_metrics WHERE source_module = 'docker' "
+                "AND resource_name = :r"), {"r": f"dsk-{hid[:8]}"})).all()
+            return summary, img_count, [r[0] for r in after], reclaimable2, metrics
 
-    summary, img_count, after_ids, reclaimable2 = asyncio.run(_go())
+    summary, img_count, after_ids, reclaimable2, metrics = asyncio.run(_go())
+    assert {v for m, v in metrics if m == "disk_total_bytes"} == {1155.0}       # 1000 + 100 + 40 + 15
+    assert sorted(v for m, v in metrics if m == "disk_reclaimable_bytes") == [0.0, 700.0]
+    assert {m for m, _ in metrics} == {"disk_images_bytes", "disk_containers_bytes", "disk_volumes_bytes",
+                                       "disk_build_cache_bytes", "disk_total_bytes", "disk_reclaimable_bytes"}
     assert summary[0] == 700 and summary[1] == 2 and summary[2] == 15
     assert img_count == 2
     assert after_ids == ["aaaaaaaaaaaa"]   # the unreferenced image pruned on re-report