Inconsistent subcategories naming in backend models

[jackieo5023]

Describe the bug
If you look at our category model, you see "name" and "subcategories" fields, which makes sense for an array of subcategories. However, if you look at our resource model, you see "subcategory" as a field, but it is an array of strings.

Expected behavior
The "subcategory" field in Resource should be renamed to "subcategories" to be more consistent and make logical sense.
We may also need to edit some backend endpoints to reflect this change.