feat(host_agent): each drive's health, read with smartctl, alertable (#5789)

The root helper gains one read-only action, `drives`: it finds the drives
itself (smartctl --scan-open), reads each as JSON, and writes a summary the
agent carries on every sample (agent 1.20.0). Steward judges the figures in
drives.py (failing / worn / watch / ok / no SMART / unreadable), records them
per drive and as host roll-ups for covering rules, keeps the newest report on
the registration and lists it on the host page.

install-deps.sh installs smartmontools with apt, dnf, yum, zypper, pacman or
apk, run by install.sh and every agent playbook; one it can't install is
reported, never fatal.

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

plugins/host_agent/models.py

@@ -2,7 +2,7 @@
 from __future__ import annotations
 import uuid
 from datetime import datetime, timezone
-from sqlalchemy import Boolean, Column, String, DateTime, Float, ForeignKey, Integer, Text
+from sqlalchemy import JSON, Boolean, Column, String, DateTime, Float, ForeignKey, Integer, Text
 from steward.models.base import Base
 
 
@@ -51,6 +51,11 @@ class HostAgentRegistration(Base):
     # its request directory) that updates and reboots go through (#5766); None
     # until an agent that says reports.
     maint_helper = Column(Boolean, nullable=True)
+    # The host's newest drive-health report, as the root helper read it with
+    # smartctl (agent ≥ 1.20.0, #5789): {"checked_at", "smartctl", "drives"} or
+    # {"error"}. drives.py judges it; the figures are also metrics.
+    drive_health = Column(JSON, nullable=True)
+    drive_health_at = Column(DateTime(timezone=True), nullable=True)
     created_at = Column(DateTime(timezone=True), nullable=False, default=_utcnow)
     updated_at = Column(DateTime(timezone=True), nullable=False,
                         default=_utcnow, onupdate=_utcnow)