The Reports page expects structured objects from the API (e.g. summary
with nested revenue/appointments fields, revenue with byPeriod/byGroomer,
etc.). Returning a bare [] caused runtime errors when the component
accessed properties like apptData.byPeriod, crashing the React tree and
making "Groom Book" disappear from the DOM on retries.
Co-Authored-By: Paperclip <noreply@paperclip.ing>