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>

docs/plugins/prometheus.md

@@ -48,6 +48,8 @@ A value's unit is read from Prometheus naming: `_bytes` as bytes, `_seconds` as
 
 **A refused token stops the scraping.** When an endpoint answers 401 or 403, Steward stops asking and says so on the target's page and in the overview, because asking again won't change the answer and some services, Home Assistant among them, ban an address after a few refusals. Saving the target, usually with a new token, or pausing and resuming it, starts it again. Any other failure is retried at the next scrape.
 
+**A target can use a plugin's sign-in instead of a token.** Home Assistant's page sets this up, so one Home Assistant user covers its metrics too. A token typed into the target's form replaces it. That sign-in is only sent to the address it was made for, and a refusal stops it from the plugin's page, not here.
+
 A **state set** is one series per possible state, the current one 1 and the rest 0, such as `fabledarchives_backup_target_state{target, state}`. The target page shows it as one row per target (or job), naming the current state; the other states are listed on hover. A metric is read as a state set when its name ends in `_state` and every series has a `state` label reading 0 or 1. The OpenMetrics form, where the label is named after the metric (`foo{foo="a"} 1`), counts too. Each state is still stored as its own series, so an alert rule can watch one state.
 
 **Only what the endpoint reports now is current.** When a scrape succeeds, any series it no longer has loses its newest value. That happens when the endpoint stopped reporting it, for example an old `build_info` after an upgrade, or when a filter or the cap now leaves it out. Its history stays on the chart until retention removes it. A failed scrape leaves the newest values as they were.