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/routes/public.py

@@ -40,12 +40,13 @@ def _get_cached_notes():
     Get cached note list or fetch fresh notes
 
     Returns cached notes if still valid, otherwise fetches fresh notes
-    from database and updates cache. Includes media for each note.
+    from database and updates cache. Includes media and tags for each note.
 
     Returns:
-        List of published notes for feed generation (with media attached)
+        List of published notes for feed generation (with media and tags attached)
     """
     from starpunk.media import get_note_media
+    from starpunk.tags import get_note_tags
 
     # Get cache duration from config (in seconds)
     cache_seconds = current_app.config.get("FEED_CACHE_SECONDS", 300)
@@ -68,6 +69,10 @@ def _get_cached_notes():
         media = get_note_media(note.id)
         object.__setattr__(note, 'media', media)
 
+        # Attach tags to each note (v1.3.1)
+        tags = get_note_tags(note.id)
+        object.__setattr__(note, 'tags', tags)
+
     _feed_cache["notes"] = notes
     _feed_cache["timestamp"] = now