feat(homeassistant): an admin signs in to Home Assistant, with nothing added to it (#6355)

Replaces the read-only token and the webhook automation, which every install
would have had to paste into its own Home Assistant. Connect now goes to Home
Assistant's own sign-in page, and Steward keeps the refresh token that comes
back, encrypted, and calls backup.create_automatic and update.install directly.

The connection page warns that the stored credential has the same access as
the user who signed in, and recommends a service account so Steward's actions
are easy to tell apart in Home Assistant's logs. A user who isn't an
administrator gets the status page and no Install.

Those actions answer only when finished, so a request cut off once sent counts
as sent and the outcome is read from the entities. Removing the connection, or
signing in again, ends the old sign-in on Home Assistant.

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

steward/audit/service.py

@@ -214,7 +214,7 @@ 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_saved": "signed Steward in to Home Assistant",
     "homeassistant.connection_removed": "removed the Home Assistant connection",
     "homeassistant.update_started": "started a Home Assistant update",
     "syslog.source_added": "added syslog source",