Add panel-plane API client with config plumbing (Feature 18)

The integration will consume the NestQuest API service (Feature 16) over
HTTP instead of reading the database. This adds the one module that
speaks the panel plane: api_client.py with get_snapshot() and
complete_instance(), a typed NestQuestApiError for transport failures
and non-2xx responses (carrying the HTTP status so 401/404 are
distinguishable), one timeout per request and no retry loops. The HTTP
session is injected, so tests exercise the client against a stub
transport with no network and no aiohttp; the production factory hands
in HA's shared aiohttp session and resolves the base URL and panel
service token off the config entry (options, then entry data, then
defaults). The two new settings are wired into the options flow like
every other section (validation mirrored with the client's constructor
rules); the token is a secret and is never logged or echoed into error
messages. Coordinator wiring is the next task.

custom_components/nestquest/core/const.py

@@ -101,6 +101,23 @@ CONF_END_OF_DAY_REPORT_ENABLED = "end_of_day_report_enabled"
 CONF_CELEBRATION_ENABLED = "celebration_enabled"
 DEFAULT_AUTOMATION_ENABLED = True
 
+# Panel-plane API client (Feature 18): the integration reaches the
+# NestQuest API service (Feature 16) over HTTP instead of the database.
+# The base URL points at the API service host serving ``/api/v1/panel/*``
+# (uvicorn's default host/port is the sensible starting point); the panel
+# service token is the static Bearer credential the panel plane requires
+# (``NESTQUEST_PANEL_TOKEN`` on the API side).  Both are stored on the
+# entry like every other setting — options first, then the entry-data
+# copy, then defaults.  The TOKEN IS A SECRET: it is never logged, never
+# echoed into error messages, and never committed.  No invented default
+# token ships (an empty stored token means "not configured yet", and the
+# panel plane answers 401 until a real one is saved) — mirroring the API
+# service's fail-closed stance (api/config.py: no default token exists).
+CONF_API_BASE_URL = "api_base_url"
+DEFAULT_API_BASE_URL = "http://127.0.0.1:8000"
+CONF_PANEL_TOKEN = "panel_token"
+DEFAULT_PANEL_TOKEN = ""
+
 # Quest windows (D-008): a definition may declare several day windows;
 # each produces its own instance per day and groups the panel's Quest
 # Log into three columns.  The clock ranges are stored here so they stay