docs: Add v1.3.1 and v1.4.0 release definitions

v1.3.1 "Syndicate Tags":
- RSS/Atom/JSON Feed category/tag support

v1.4.0 "Media":
- Micropub media endpoint (W3C compliant)
- Large image support (>10MB auto-resize)
- Enhanced feed media (image variants, full Media RSS)

Also adds tag-filtered feeds to backlog at medium priority.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

docs/projectplan/BACKLOG.md

@@ -29,7 +29,7 @@
 
 ## High
 
-### Enhanced Feed Media Support
+### Enhanced Feed Media Support *(Scheduled: v1.4.0)*
 - Multiple image sizes/thumbnails (150px, 320px, 640px, 1280px)
 - Full Media RSS implementation (media:group, all attributes)
 - Enhanced JSON Feed attachments