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>

plugins/prometheus/service.py

@@ -212,7 +212,7 @@ def form_values(target: PrometheusTarget | None) -> dict:
     return {"name": target.name, "url": target.url, "interval_seconds": target.interval_seconds,
             "series_cap": target.series_cap, "verify_tls": target.verify_tls,
             "keep": "\n".join(target.keep or []),
-            "drop": "\n".join(target.drop or []), "has_token": bool(target.token)}
+            "drop": "\n".join(target.drop or []), "has_token": bool(target.token), "token_from": target.token_from}
 
 
 def _bounded(form, key: str, default: int, low: int, high: int, says: str) -> int:
@@ -250,6 +250,7 @@ def parse_form(form, existing: PrometheusTarget | None = None) -> dict:
     else:
         token = ""
     return {"name": name, "url": url, "token": token,
+            "token_from": None if typed or existing is None else existing.token_from,
             "verify_tls": bool(form.get("verify_tls")), "interval_seconds": interval,
             "keep": patterns(form.get("keep") or ""), "drop": patterns(form.get("drop") or ""),
             "series_cap": cap}