Retrieve or update push notification category preferences for the current user.

GET — Returns the current toggle values. Lazily creates a PushPreference row with default values (both toggles True) if none exists yet.

PATCH — Accepts a partial payload and updates only the supplied boolean fields. Unknown fields in the payload are ignored to allow forward-compatible client implementations.

GET response / PATCH response: { "activity_enabled": <bool>, "editorial_enabled": <bool> }