PRICE-01: price_service provider, fallback chain, and the gate read from it

A new price source, `price_provider: price_service`: the owner's price server,
one fixed HTTPS host, a bearer token (new password option
`price_service_token`), wholesale energy only so markup and taxes are added as
for generic_dayahead.

The chain is server, then energy-charts, then the prices the history store
kept. A step counts only if it has a price for now. Each step down and back up
is one log line. With every source down a site plans from what it saved.

No price for now: one status row, one Home plan line and one notification, in
English and Dutch, cleared when a price returns. With this provider the
maintenance charge gate reads the provider's prices and keeps the Home
Assistant rule: the current 15-minute price is at or below today's average.

The token is hidden on the settings page, refused when missing, and taken out
of any error text. Other providers, `off` and the Home Assistant pair take the
road they always took (scripted-day probe against 0ce7d35: one line differs,
the refusal that lists the known providers). See
DECISIONS/PRICE-01-own-price-server.md.

Co-Authored-By: Claude <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Bdqr7PiaHcBnzPituu2yTC

goodwe_controller/app/uipage.py

@@ -18,8 +18,10 @@ from . import uimodel as U
 from .configui import help_of, label_of, problems_by_field, split_group
 from .validate import power_request_entities
 
-# A choice the page cannot make, only keep. None since P5's period editor.
-ONLY_IF_HELD = ()
+# A choice the page cannot make, only keep. `price_service` needs a token, and
+# the token is a secret this page never draws (uilayout.HIDDEN), so it is set on
+# the Configuration tab and the page only keeps it (PRICE-01).
+ONLY_IF_HELD = ("price_service",)
 _TIERS = ("bad", "warn", "info")