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/audit/service.py

@@ -214,6 +214,9 @@ ACTIONS = {
     "prometheus.template_created": "added Prometheus widget template",
     "prometheus.template_updated": "changed Prometheus widget template",
     "prometheus.template_deleted": "deleted Prometheus widget template",
+    "homeassistant.connection_saved": "saved the Home Assistant connection",
+    "homeassistant.connection_removed": "removed the Home Assistant connection",
+    "homeassistant.update_started": "started a Home Assistant update",
     "syslog.source_added": "added syslog source",
     "syslog.source_updated": "changed syslog source",
     "syslog.source_removed": "removed syslog source",
@@ -244,7 +247,7 @@ AREAS = (("auth.", "Sign-ins"), ("account.", "Accounts"), ("user.", "Users"), ("
          ("ansible.run", "Playbook runs"), ("ansible.", "Ansible (all)"),
          ("settings.", "Settings"), ("plugin.", "Plugins"), ("api_token.", "AI access"),
          ("docker.", "Docker"), ("snmp.", "SNMP"), ("prometheus.", "Prometheus"),
-         ("syslog.", "Syslog"), ("backup.", "Backups"), ("timeline.", "Timeline"),
+         ("syslog.", "Syslog"), ("homeassistant.", "Home Assistant"), ("backup.", "Backups"), ("timeline.", "Timeline"),
          ("ticket.", "Tickets"))