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/backup/contents.py

@@ -29,6 +29,7 @@ KEPT: frozenset[str] = frozenset({
     # Syslog senders: which were added, under what name, and which ignored.
     "syslog_sources",
     "syslog_patterns",
+    "homeassistant_connection",
 })
 
 # Rows that are measurements, logs, history or in-flight work: left out.
@@ -67,6 +68,7 @@ EXCLUDED: frozenset[str] = frozenset({
     "unifi_events", "unifi_fw_rules", "unifi_networks", "unifi_port_forwards",
     "unifi_speedtest_results", "unifi_wan_stats", "unifi_wlan_stats",
     "syslog_lines",
+    "homeassistant_updates",
 })
 
 # ── What a check compares (#5823) ────────────────────────────────────────────
@@ -91,6 +93,8 @@ ACTIVITY: dict[str, frozenset[str]] = {
     "snmp_devices": frozenset({"last_polled_at", "last_ok_at", "last_error", "missing"}),
     "prometheus_targets": frozenset({"last_scraped_at", "last_ok_at", "last_error", "last_kept",
                                      "last_dropped", "last_capped"}),
+    "homeassistant_connection": frozenset({"refused", "last_polled_at", "last_ok_at", "last_error",
+                                           "updates", "backup", "automation"}),
     # What the sender last sent; its address, name, state and host are setup.
     "syslog_sources": frozenset({"hostname_seen", "first_seen_at", "last_seen_at", "lines_total",
                                  "last_severity", "last_line"}),
@@ -121,5 +125,6 @@ AREAS: tuple[tuple[str, tuple[str, ...]], ...] = (
     ("Ansible", ("ansible_targets", "ansible_groups", "ansible_target_groups", "ansible_schedules")),
     ("Dashboards", ("dashboards", "dashboard_widgets", "dashboard_share_tokens")),
     ("Plugins", ("snmp_devices", "prometheus_targets", "prometheus_widget_templates",
-                 "unifi_known_clients", "syslog_sources", "syslog_patterns")),
+                 "unifi_known_clients", "syslog_sources", "syslog_patterns",
+                 "homeassistant_connection")),
 )