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>

plugins/homeassistant/plugin.yaml

@@ -0,0 +1,17 @@
+# plugins/homeassistant/plugin.yaml
+name: homeassistant
+version: "1.0.0"
+description: "Shows which Home Assistant updates are waiting and how old its last backup is, and installs Core, OS and add-on updates after a confirmed backup."
+author: "Steward"
+license: "MIT"
+min_app_version: "0.1.0"
+repository_url: "https://git.fabledsword.com/bvandeusen/FabledSteward"
+homepage: "https://git.fabledsword.com/bvandeusen/FabledSteward/src/branch/dev/plugins/homeassistant"
+tags:
+  - home-assistant
+  - updates
+  - backups
+
+# The connection is added and edited on the Home Assistant page
+# (/plugins/homeassistant/).
+config: {}