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/models.py

@@ -189,6 +189,56 @@ class DockerSwarmService(Base):
     )
 
 
+class DockerDiskUsage(Base):
+    """Per-host Docker disk usage summary from `/system/df` (current state).
+
+    One row per host (the agent re-reports the full summary each interval).
+    Reclaimable = bytes held by images no container references. Sizes are bytes
+    (BigInteger — image caches exceed 2^31 easily).
+    """
+    __tablename__ = "docker_disk_usage"
+
+    host_id: Mapped[str] = mapped_column(
+        String(36), ForeignKey("hosts.id", ondelete="CASCADE"), primary_key=True
+    )
+    layers_size: Mapped[int] = mapped_column(BigInteger, nullable=False, default=0)
+    images_size: Mapped[int] = mapped_column(BigInteger, nullable=False, default=0)
+    images_reclaimable: Mapped[int] = mapped_column(BigInteger, nullable=False, default=0)
+    containers_size: Mapped[int] = mapped_column(BigInteger, nullable=False, default=0)
+    volumes_size: Mapped[int] = mapped_column(BigInteger, nullable=False, default=0)
+    build_cache_size: Mapped[int] = mapped_column(BigInteger, nullable=False, default=0)
+    images_total: Mapped[int] = mapped_column(Integer, nullable=False, default=0)
+    images_active: Mapped[int] = mapped_column(Integer, nullable=False, default=0)
+    containers_count: Mapped[int] = mapped_column(Integer, nullable=False, default=0)
+    volumes_count: Mapped[int] = mapped_column(Integer, nullable=False, default=0)
+    updated_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), nullable=False,
+        default=lambda: datetime.now(timezone.utc),
+    )
+
+
+class DockerImage(Base):
+    """Per-host image storage record (the heavy hitters from `/system/df`).
+
+    Keyed (host_id, image_id); `containers` is the reference count (0 ⇒ the
+    image's size is reclaimable). Replaced wholesale per host on each report.
+    """
+    __tablename__ = "docker_images"
+
+    host_id: Mapped[str] = mapped_column(
+        String(36), ForeignKey("hosts.id", ondelete="CASCADE"), primary_key=True
+    )
+    image_id: Mapped[str] = mapped_column(String(64), primary_key=True)
+    repo_tag: Mapped[str] = mapped_column(String(512), nullable=False, default="<none>")
+    size: Mapped[int] = mapped_column(BigInteger, nullable=False, default=0)
+    shared_size: Mapped[int] = mapped_column(BigInteger, nullable=False, default=0)
+    containers: Mapped[int] = mapped_column(Integer, nullable=False, default=0)
+    updated_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), nullable=False,
+        default=lambda: datetime.now(timezone.utc),
+    )
+
+
 class DockerSwarmNode(Base):
     """A Swarm node as seen by a manager host: role / availability / status."""
     __tablename__ = "docker_swarm_nodes"