[Blog] RSS feed

## Summary
Generate RSS feed for blog posts.

## Technical Details
- RSS 2.0 format
- Include title, description, link, pubDate
- Auto-generate at build time

## Files
- `apps/marketing/src/app/blog/rss.xml/route.ts`

## Acceptance Criteria
- [ ] RSS feed at /blog/rss.xml
- [ ] All posts included
- [ ] Valid RSS format
- [ ] Enclosure for images if applicable