feat(docker): Docker's own status reaches Steward — an empty answer clears a host, a lost one greys it, a demoted manager's swarm rows go (#5152)

The agent (1.9.0) said nothing when Docker could not be asked and nothing
when Docker had no containers, so Steward could tell neither apart from the
other, and kept a host's last containers on screen as running either way.

- Agent: collect_docker returns (status, detail, containers). "ok" sends
  docker: [] when there are none; "unavailable" (no socket, nothing
  listening) and "error" (permission, timeout, bad answer) say why.
  collect_swarm says whether /info answered, and a node that is not a
  manager sends swarm_manager: false. A failed /info says nothing.
- Ingest: an empty docker list is a snapshot; the newest sample's status
  goes to the Docker plugin with the rest.
- docker_host_status (docker_011): per host, status, detail, since and
  last report. A host that never had Docker gets no row.
- An "ok" empty snapshot reaps the host's containers; an error keeps them.
  swarm_manager: false drops the host's swarm services and nodes.
- Docker page: a "Docker not answering" card names each such host, why,
  and since when; its containers read as stale. Duplicate header comment
  in rows.html removed.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

plugins/docker/models.py

@@ -304,3 +304,24 @@ class DockerSwarmNode(Base):
         DateTime(timezone=True), nullable=False,
         default=lambda: datetime.now(timezone.utc),
     )
+
+
+class DockerHostStatus(Base):
+    """Whether Docker answered on a host at its agent's last report (agent ≥ 1.9.0).
+
+    "ok" when it did; "unavailable" when there is no Docker to ask (no socket,
+    nothing listening); "error" when Docker is there and the call failed.
+    `detail` says why for the last two. While a host is not "ok" its container
+    rows are what Docker last said, not what is true now, and read as stale.
+    A host that never had Docker gets no row.
+    """
+    __tablename__ = "docker_host_status"
+
+    host_id: Mapped[str] = mapped_column(
+        String(36), ForeignKey("hosts.id", ondelete="CASCADE"), primary_key=True
+    )
+    status: Mapped[str] = mapped_column(String(16), nullable=False)
+    detail: Mapped[str | None] = mapped_column(String(255), nullable=True)
+    # When the status last changed, and the agent's newest report.
+    since: Mapped[datetime] = mapped_column(DateTime(timezone=True), nullable=False)
+    reported_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), nullable=False)