Serve the panel snapshot behind a reusable service-token dependency (task ae7e90ae)

The panel plane (D-012) needs an authenticated read surface the HA
integration polls: GET /api/v1/panel/snapshot. The token check lives in
ONE reusable dependency (api/dependencies.require_panel_token) — missing,
malformed, wrong, and empty-configured credentials all fail closed with
the same 401 (constant-time compare, uniform detail) — and the panel
router attaches it once so every later panel route inherits the check.

ApiConfig gains the required NESTQUEST_PANEL_TOKEN (same fail-fast
discipline as NESTQUEST_DB_PATH: unset/empty is a ConfigError, never a
silent default), create_app stores the resolved config on app.state so
the dependency reads it without module state, and the snapshot handler
stays a thin adapter: it calls the pure core build_snapshot with the
live database and one timezone-aware clock read, shapes each child's
instances through core.instance_payload(include_missed=False) so missed
instances are omitted from the panel payload (D-009), and returns the
shape documented by the PanelSnapshotResponse models in OpenAPI.

Tests (tests/test_api_panel.py) seed the household through the SAME
nestquest_core.* copy the app uses, pin the route's single clock read,
and assert 401 for missing/wrong/malformed tokens, 200 with the full
documented payload (children in sort order, counts, presence,
cycle_day), and that a past-due open (missed) instance never reaches
the payload.

api/nestquest_core.py

@@ -107,4 +107,19 @@ core: ModuleType = _load_core()
 core_db = importlib.import_module(f"{_CORE_MODULE_NAME}.db")
 core_migrations = importlib.import_module(f"{_CORE_MODULE_NAME}.migrations")
 
-__all__ = ["core", "core_db", "core_migrations"]
+#: The panel snapshot builder and its payload shaper
+#: (``core.snapshot.build_snapshot`` / ``instance_payload``) — the pure
+#: assembly the panel routes call.
+core_snapshot = importlib.import_module(f"{_CORE_MODULE_NAME}.snapshot")
+
+#: The explicit settings object (``core.settings.NestQuestSettings``)
+#: the snapshot builder's ``settings`` parameter takes.
+core_settings = importlib.import_module(f"{_CORE_MODULE_NAME}.settings")
+
+__all__ = [
+    "core",
+    "core_db",
+    "core_migrations",
+    "core_snapshot",
+    "core_settings",
+]