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/migrations/versions/prometheus_004_token_from.py

@@ -0,0 +1,23 @@
+"""A target can take its token from another plugin's sign-in
+
+Revision ID: prometheus_004_token_from
+Revises: prometheus_003_refused
+Create Date: 2026-11-01
+"""
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+from alembic import op
+
+revision: str = "prometheus_004_token_from"
+down_revision: Union[str, None] = "prometheus_003_refused"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.add_column("prometheus_targets", sa.Column("token_from", sa.String(50), nullable=True))
+
+
+def downgrade() -> None:
+    op.drop_column("prometheus_targets", "token_from")