feat(feed): add external tag to linkposts in RSS feeds

[Xe]

Summary

Closes SITE-9

This change adds support for automatically tagging linkposts (posts with redirect_to field) with an "external" tag in both RSS and JSON feeds. This allows readers to perform client-side filtering to identify external content.

Changes Made

• Added categories field to FeedItem interface in the feed plugin
• Enhanced feed item creation to detect linkposts via redirect_to field
• Automatically append "external" tag to linkposts while preserving any existing tags
• Include categories in RSS feed as <category> elements
• Include categories in JSON feed as tags array

Test Plan

• Verified that linkposts (posts with redirect_to field) correctly receive the <category>external</category> tag in RSS feeds
• Verified that regular posts do not receive the external tag
• Verified that JSON feeds include the "external" tag in the tags array for linkposts
• Confirmed that existing tags are preserved when present
• Tested with both internal and external URLs

Implementation Details

The implementation works by:

1. Detecting if a blog post has a redirect_to field in its frontmatter
2. Getting any existing tags from the post's frontmatter
3. Appending the "external" tag to the existing tags for linkposts
4. Including the categories in both RSS and JSON feed formats

This allows feed consumers to programmatically identify external content for filtering or display purposes.

[linear]

SITE-9 Linkposts should have an "external" tag in the RSS feed