Add featured image, tags, and categories to RSS feed generation

[bencounsell]

Summary

Implements RSS feed generation that includes featured images, tags, and categories from markdown frontmatter — addressing the customer request in the forum thread.

Related to #71

What changed

New: RSS feed builder (shared/rss/rss-item-builder.js)

A JavaScript RSS 2.0 feed builder that generates standards-compliant XML with support for:

• Featured images via three complementary RSS elements:
  • <enclosure> — standard RSS 2.0 image attachment
  • <media:content> — Media RSS (Yahoo mrss) for richer feed readers
  • <media:thumbnail> — Media RSS thumbnail hint
• Tags via <category> elements (one per tag)
• Categories via <category> elements (one per category)
• Atom self-link for feed auto-discovery

The builder accepts the image frontmatter key in both forms used by the CMS:

• String URL: image: "https://example.com/hero.jpg"
• Object with src: image: { src: "https://example.com/hero.jpg" }

New: PHP feed template (shared/templates/rss/feed.php)

A server-side PHP template that reads markdown files with YAML frontmatter from a CMS content directory and produces RSS 2.0 XML. Follows the same pattern as the gallery's remote-folder PHP templates.

New: Tests (test/rss-feed.test.mjs)

23 tests covering:

• XML escaping for all five special characters
• RFC 822 date conversion
• Image URL resolution (string and object forms)
• MIME type detection for all common image formats
• <enclosure>, <media:content>, <media:thumbnail> emission for featured images
• <category> emission for string tags, enriched tag objects, and categories
• Combined tags + categories output
• Full feed structure with Media RSS and Atom namespaces
• End-to-end round-trip with generated XML fixture

Frontmatter keys

All optional. These reuse existing CMS markdown collection conventions — no new keys were invented for fields the CMS already supports.

Key	Type	Description
title	string	Post title
date_published	string	Publication date (ISO 8601 or any parseable format)
description	string	Plain-text excerpt or summary
image	string or { src }	Featured/header image URL
tags	string[] or { title, url }[]	Tag labels (emitted as <category>)
categories	string[]	Category labels (emitted as <category>)
link	string	Item permalink
guid	string	Globally unique identifier (defaults to link)

Test results

# tests 115
# pass 115
# fail 0

All 23 new RSS tests pass alongside the 92 existing component tests.