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/migrations/versions/docker_011_host_status.py

@@ -0,0 +1,36 @@
+"""Docker host status
+
+Adds docker_host_status: per host, whether Docker answered at the agent's
+last report (ok / unavailable / error) and why not. Lets Steward show a host
+whose Docker went away as such, instead of its last containers as running
+(#5152). Additive create_table.
+
+Revision ID: docker_011_host_status
+Revises: docker_010_swarm_tasks
+Create Date: 2026-10-06
+"""
+from typing import Sequence, Union
+from alembic import op
+import sqlalchemy as sa
+
+revision: str = "docker_011_host_status"
+down_revision: Union[str, None] = "docker_010_swarm_tasks"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.create_table(
+        "docker_host_status",
+        sa.Column("host_id", sa.String(length=36), nullable=False),
+        sa.Column("status", sa.String(length=16), nullable=False),
+        sa.Column("detail", sa.String(length=255), nullable=True),
+        sa.Column("since", sa.DateTime(timezone=True), nullable=False),
+        sa.Column("reported_at", sa.DateTime(timezone=True), nullable=False),
+        sa.ForeignKeyConstraint(["host_id"], ["hosts.id"], ondelete="CASCADE"),
+        sa.PrimaryKeyConstraint("host_id"),
+    )
+
+
+def downgrade() -> None:
+    op.drop_table("docker_host_status")