feat: v1.4.0 Phase 4 - Enhanced Feed Media

Implement Phase 4 of v1.4.0 Media release - Enhanced Feed Media support.

RSS Feed Enhancements (starpunk/feeds/rss.py):
- Wrap size variants in <media:group> elements
- Add <media:content> for large/medium/small variants with attributes:
  url, type, medium, isDefault, width, height, fileSize
- Add <media:thumbnail> for thumb variant with dimensions
- Add <media:title type="plain"> for image captions
- Implement isDefault logic: largest available variant (large→medium→small fallback)
- Maintain backwards compatibility for media without variants (legacy fallback)

JSON Feed Enhancements (starpunk/feeds/json_feed.py):
- Add _starpunk.about URL (configurable via STARPUNK_ABOUT_URL config)
- Add _starpunk.media_variants array with variant data when variants exist
- Each variant entry includes: url, width, height, size_in_bytes, mime_type

ATOM Feed Enhancements (starpunk/feeds/atom.py):
- Add title attribute to enclosure links for captions
- Keep simple (no variants in ATOM per design decision)

Test Updates (tests/test_feeds_rss.py):
- Update streaming media test to search descendants for media:content
- Now inside media:group for images with variants (v1.4.0 behavior)

Per design document: /docs/design/v1.4.0/media-implementation-design.md
Following ADR-059: Full Feed Media Standardization

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

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

starpunk/feeds/atom.py

@@ -184,12 +184,18 @@ def generate_atom_streaming(
                 yield f'    <category term="{_escape_xml(tag["name"])}" label="{_escape_xml(tag["display_name"])}"/>\n'
 
         # Media enclosures (v1.2.0 Phase 3, per Q24 and ADR-057)
+        # Enhanced with title attribute for captions (v1.4.0 Phase 4)
         if hasattr(note, 'media') and note.media:
             for item in note.media:
                 media_url = f"{site_url}/media/{item['path']}"
                 mime_type = item.get('mime_type', 'image/jpeg')
                 size = item.get('size', 0)
-                yield f'    <link rel="enclosure" type="{_escape_xml(mime_type)}" href="{_escape_xml(media_url)}" length="{size}"/>\n'
+                caption = item.get('caption', '')
+
+                # Include title attribute for caption
+                title_attr = f' title="{_escape_xml(caption)}"' if caption else ''
+
+                yield f'    <link rel="enclosure" type="{_escape_xml(mime_type)}" href="{_escape_xml(media_url)}" length="{size}"{title_attr}/>\n'
 
         # Content - include media as HTML (per Q24)
         if note.html: