feat(homeassistant): waiting updates, backup age, and updates installed after a confirmed backup (#6355)

A new plugin reads Home Assistant with a read-only token every 5 minutes and
shows which Core, OS and add-on updates are waiting and how old the last backup
is. Install takes a backup through a local-only webhook, confirms Home
Assistant's last-backup time moved, installs, and follows the update to its end,
through Home Assistant's and Steward's own restarts. One runs at a time. Each
ends as one timeline entry.

Steward holds nothing that can act on the house: the webhook's automation can
only take a backup or install an allowed update (System 231's ruling). A refused
token stops polling, as Home Assistant bans an address after a few refusals.

Milestone 576 steps 1 to 5.

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

steward/core/settings.py

@@ -181,6 +181,7 @@ DEFAULTS: dict[str, Any] = {
     "plugin.snmp": {"enabled": True},
     "plugin.prometheus": {"enabled": True},
     "plugin.syslog": {"enabled": True},
+    "plugin.homeassistant": {"enabled": True},
     # OIDC single-sign-on
     "oidc.enabled": False,
     "oidc.discovery_url": "",