feat: Add tag/category support to all feed formats (v1.3.1)

Implements tag/category rendering in RSS 2.0, Atom 1.0, and JSON Feed 1.1
syndication feeds. Tags added in v1.3.0 are now visible to feed readers.

Changes:
- Load tags in feed generation (_get_cached_notes)
- Add <category> elements to RSS 2.0 feeds
- Add <category term/label> to Atom 1.0 feeds
- Add "tags" array to JSON Feed 1.1 items
- Omit empty tags field in JSON Feed (minimal approach)

Standards compliance:
- RSS 2.0: category element with display name
- Atom 1.0: RFC 4287 category with term and label attributes
- JSON Feed 1.1: tags array of strings

Per design: docs/design/v1.3.1/feed-tags-design.md
Implementation: docs/design/v1.3.1/2025-12-10-feed-tags-implementation.md

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

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

starpunk/feeds/rss.py

@@ -142,6 +142,11 @@ def generate_rss(
         # feedgen automatically wraps content in CDATA for RSS
         fe.description(html_content)
 
+        # Add category elements for tags (v1.3.1)
+        if hasattr(note, 'tags') and note.tags:
+            for tag in note.tags:
+                fe.category({'term': tag['display_name']})
+
         # Add RSS enclosure element (first image only, per RSS 2.0 spec)
         if hasattr(note, 'media') and note.media:
             first_media = note.media[0]
@@ -293,6 +298,12 @@ def generate_rss_streaming(
         item_xml += f"""
       <description><![CDATA[{html_content}]]></description>"""
 
+        # Add category elements for tags (v1.3.1)
+        if hasattr(note, 'tags') and note.tags:
+            for tag in note.tags:
+                item_xml += f"""
+      <category>{_escape_xml(tag['display_name'])}</category>"""
+
         # Add media:content elements (all images)
         if hasattr(note, 'media') and note.media:
             for media_item in note.media: