NewBook credentials from central Settings service

Stack-wide NewBook config lives in the Settings app (LXC 116) and is
fetched live via SETTINGS_URL/SETTINGS_SECRET — same pattern as cashup,
room-planner and maintenance. App-local system_config credentials remain
as a fallback for standalone/dev use. The app's Settings → Newbook page
no longer edits credentials; it points to the central app and keeps
Test Connection.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/services/newbook_client.py

@@ -48,7 +48,17 @@ class NewbookClient:
 
     @classmethod
     async def from_db(cls, db):
-        """Create client with credentials from database"""
+        """
+        Create client with credentials from the central Settings service
+        (stack-wide NewBook config), falling back to the app-local
+        system_config table for standalone/dev use.
+        """
+        from services.central_settings import get_newbook_credentials
+
+        central = await get_newbook_credentials()
+        if central:
+            return cls(**central)
+
         from api.config import _get_config_value
 
         api_key = await _get_config_value(db, "newbook_api_key")