feat(host_agent): Steward hands a host a maintenance job and follows it through the reboot (#5767)

#5767 "Steward hands a host a maintenance job and follows it to the end,
through its own restarts", milestone 514.

A host_maintenance_jobs row per job: queued → updating → [draining →]
rebooting → [undraining →] done, or failed / timed_out. Steward's answer to
an ingest post carries the step that host's agent should carry out now; the
agent (1.18.0) leaves each step id for the root helper once, and the helper
refuses an id it already ran, so repeats are harmless. Results come back as
the #5766 maintenance events, matched by step id; the reboot completes when
the host's `started` event has a boot_at after the reboot was handed out.

The job reboots only when the update says one is needed (or never, if asked).
A swarm node is drained first by itself if it's a manager or by a manager
with the helper, and made active after; with no manager to drain it the job
is refused. A minute tick times out a job past its state's deadline. A
maintenance window over host_agent/<host> lasts the rest of the job.

Everything is on the row, so a Steward restart, or the reboot of the host
Steward runs on, resumes from the next post. Ingest runs it in its own
savepoint so a job going wrong never costs the samples.

The Agent panel leads with a running job and says what a finished one did;
Manage agent has Update now (with "never reboot"), or says why a job can't
start. Integration test: hand off, Steward restarts, the post-boot events
close the job.

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

plugins/host_agent/migrations/versions/host_agent_007_maint_jobs.py

@@ -0,0 +1,46 @@
+"""host_agent: a host's maintenance job, followed through its reboot (#5767)
+
+Revision ID: host_agent_007_maint_jobs
+"""
+from typing import Sequence, Union
+from alembic import op
+import sqlalchemy as sa
+
+revision: str = "host_agent_007_maint_jobs"
+down_revision: Union[str, None] = "host_agent_006_maint_helper"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.create_table(
+        "host_maintenance_jobs",
+        sa.Column("id", sa.String(36), primary_key=True),
+        sa.Column("host_id", sa.String(36), sa.ForeignKey("hosts.id", ondelete="CASCADE"),
+                  nullable=False),
+        sa.Column("state", sa.String(16), nullable=False),
+        sa.Column("state_at", sa.DateTime(timezone=True), nullable=True),
+        sa.Column("reboot_policy", sa.String(16), nullable=False),
+        sa.Column("step_id", sa.String(64), nullable=True),
+        sa.Column("step_seen", sa.Boolean(), nullable=False, server_default=sa.false()),
+        sa.Column("node_id", sa.String(64), nullable=True),
+        sa.Column("drain_host_id", sa.String(36), sa.ForeignKey("hosts.id", ondelete="SET NULL"),
+                  nullable=True),
+        sa.Column("packages", sa.Integer(), nullable=True),
+        sa.Column("reboot_needed", sa.Boolean(), nullable=True),
+        sa.Column("back_at", sa.DateTime(timezone=True), nullable=True),
+        sa.Column("detail", sa.Text(), nullable=True),
+        sa.Column("window_id", sa.String(36),
+                  sa.ForeignKey("maintenance_windows.id", ondelete="SET NULL"), nullable=True),
+        sa.Column("created_by", sa.String(36), sa.ForeignKey("users.id", ondelete="SET NULL"),
+                  nullable=True),
+        sa.Column("created_at", sa.DateTime(timezone=True), nullable=False),
+        sa.Column("finished_at", sa.DateTime(timezone=True), nullable=True),
+    )
+    op.create_index("ix_host_maintenance_jobs_host_id", "host_maintenance_jobs", ["host_id"])
+    op.create_index("ix_host_maintenance_jobs_state", "host_maintenance_jobs", ["state"])
+    op.create_index("ix_host_maintenance_jobs_step_id", "host_maintenance_jobs", ["step_id"])
+
+
+def downgrade() -> None:
+    op.drop_table("host_maintenance_jobs")