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/models.py

@@ -13,7 +13,8 @@ from steward.models.base import Base
 class PrometheusTarget(Base):
     """A /metrics endpoint Steward scrapes, edited on the Prometheus page.
 
-    The bearer token is stored encrypted. keep and drop are lists of name
+    The bearer token is stored encrypted, or `token_from` names the plugin whose
+    sign-in supplies it at each scrape (its metrics.scrape_token). keep and drop are lists of name
     patterns ("fabledarchives_*") an admin chooses: Steward does the
     filtering, the endpoint serves everything. The last_* columns are the
     newest scrape's outcome, written by the scraper, so a page can say why a
@@ -26,6 +27,7 @@ class PrometheusTarget(Base):
     name: Mapped[str] = mapped_column(String(100), nullable=False, unique=True)
     url: Mapped[str] = mapped_column(String(500), nullable=False)
     token: Mapped[str] = mapped_column(Text, nullable=False, default="")
+    token_from: Mapped[str | None] = mapped_column(String(50), nullable=True)
     verify_tls: Mapped[bool] = mapped_column(Boolean, nullable=False, default=True)
     interval_seconds: Mapped[int] = mapped_column(Integer, nullable=False, default=60)
     keep: Mapped[list] = mapped_column(JSON, nullable=False, default=list)