feat(docker): collect + persist /system/df image/disk usage (agent 1.6.0)

Backend for the image/disk panel (milestone 77 #942). Agent gains
collect_disk_usage() — one /system/df call (gated on containers existing, so
Docker-less hosts pay nothing), surfacing reclaimable bytes (image size held by
unreferenced images), layers/containers/volumes/build-cache sizes, and the top
50 images by size. Emitted as sample["docker_disk"]; host_agent ingest tracks
the newest sample's copy and hands it to the docker capability as a 5th arg.

New current-state tables docker_disk_usage (one row/host) + docker_images
(per-host image rows), docker_007 migration; ingest upserts the summary and
replaces the image set per host (stale images pruned). Unit tests for the df
parsing/reclaimable math + build_sample gating; integration test for
persistence + image-set replacement.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016Jg27rgypiW2efULXJDtMC

plugins/docker/ingest.py

@@ -151,7 +151,57 @@ async def _persist_swarm(session, host, swarm: dict) -> None:
     await session.execute(stale_nodes)
 
 
-async def persist_host_docker(session, host, snapshots, swarm=None) -> None:
+async def _persist_disk(session, host, disk: dict) -> None:
+    """Upsert this host's /system/df summary + replace its image storage rows.
+
+    Current-state (not time-series): the agent re-reports the full summary each
+    interval, so we overwrite the single summary row and replace the image set
+    (delete rows no longer present, upsert the rest), scoped to this host.
+    """
+    from datetime import timezone
+    from .models import DockerDiskUsage, DockerImage
+
+    now = datetime.now(timezone.utc)
+
+    summary = await session.get(DockerDiskUsage, host.id)
+    if summary is None:
+        summary = DockerDiskUsage(host_id=host.id)
+        session.add(summary)
+    summary.layers_size = int(disk.get("layers_size") or 0)
+    summary.images_size = int(disk.get("images_size") or 0)
+    summary.images_reclaimable = int(disk.get("images_reclaimable") or 0)
+    summary.containers_size = int(disk.get("containers_size") or 0)
+    summary.volumes_size = int(disk.get("volumes_size") or 0)
+    summary.build_cache_size = int(disk.get("build_cache_size") or 0)
+    summary.images_total = int(disk.get("images_total") or 0)
+    summary.images_active = int(disk.get("images_active") or 0)
+    summary.containers_count = int(disk.get("containers_count") or 0)
+    summary.volumes_count = int(disk.get("volumes_count") or 0)
+    summary.updated_at = now
+
+    images = disk.get("images") or []
+    seen: set[str] = set()
+    for im in images:
+        iid = im.get("image_id")
+        if not iid or iid in seen:
+            continue
+        seen.add(iid)
+        row = await session.get(DockerImage, (host.id, iid))
+        if row is None:
+            row = DockerImage(host_id=host.id, image_id=iid)
+            session.add(row)
+        row.repo_tag = (im.get("repo_tag") or "<none>")[:512]
+        row.size = int(im.get("size") or 0)
+        row.shared_size = int(im.get("shared_size") or 0)
+        row.containers = int(im.get("containers") or 0)
+        row.updated_at = now
+    stale = delete(DockerImage).where(DockerImage.host_id == host.id)
+    if seen:
+        stale = stale.where(DockerImage.image_id.notin_(seen))
+    await session.execute(stale)
+
+
+async def persist_host_docker(session, host, snapshots, swarm=None, disk=None) -> None:
     """Upsert containers + time-series + lifecycle events + swarm for one host.
 
     `snapshots` is a list of (recorded_at: datetime, containers: list[dict]) —
@@ -160,12 +210,16 @@ async def persist_host_docker(session, host, snapshots, swarm=None) -> None:
     DockerMetric rows; the newest snapshot drives current container state, the
     alert pipeline, and lifecycle-event derivation. `swarm` is the newest
     sample's swarm object (or None off managers) — persisted when present.
+    `disk` is the newest sample's /system/df summary (or None on Docker-less
+    hosts) — persisted when present.
     """
     from steward.core.alerts import record_metric
     from .models import DockerContainer, DockerEvent, DockerMetric
 
     if swarm is not None:
         await _persist_swarm(session, host, swarm)
+    if disk is not None:
+        await _persist_disk(session, host, disk)
 
     if not snapshots:
         return