feat(homeassistant): its page offers the metrics scrape under the same sign-in, so one user covers everything (#6744)

A Prometheus target's token_from names the plugin that supplies its token at each
scrape. Read its metrics makes the target; Use this sign-in moves one at that address
across, keeping its name, filters and history. A refusal stops the connection, so one
Try again or sign-in starts everything, and the sign-in is only sent to its own address.
A 404 says how to turn on Home Assistant's Prometheus integration, as Steward doesn't
change Home Assistant. Refused also joins the volatile columns a backup check ignores.

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

steward/backup/contents.py

@@ -91,7 +91,7 @@ ACTIVITY: dict[str, frozenset[str]] = {
     "monitors": frozenset({"last_checked_at"}),
     "ansible_schedules": frozenset({"last_run_at", "last_run_id", "last_error"}),
     "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",
+    "prometheus_targets": frozenset({"refused", "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"}),