Add category Name Override and Short Name Support

Add optional configuration capabilities for primary category display customization.

## Features to Implement

### 1. Short Name
Allow categories to have an alternative "short name" that can be used consistently across templates where space is limited or a different presentation is preferred.

**Implementation Details:**
- Add a `short_name` term meta field to categories
- Field should be optional and fall back to the regular category name if not set
- Expose via REST API for block editor support
- Add a `get_short_name()` helper function

### 2. Category Override (Per-Post Level)
Enable editors to override the displayed category name on a per-post basis without changing the actual category.

**Implementation Details:**
- Add a `_category_override` post meta field
- Field should be optional and fall back to the category name (or short name if available)
- Only affects display, not the actual category assignment
- Expose via REST API for block editor support

For reference:
- this has been implemented on other projects, reach out to the Americas Squad for more details.
- It was brifly implemented on primary category at first but we decided to not go further and lunch an MVP, this is the base for it: https://github.com/humanmade/primary-category/pull/1/changes/5822eb295ec10b038da276137cf7a7c1e92a7c7e