feat: Add description field to PluginTemplateListSerializer

The description field is now included in marketplace listing responses,
allowing the frontend to display full plugin descriptions with formatting,
bullet points, and detailed benefits.

All 6 platform plugins now return complete information:
- Name, author (Smooth Schedule), version (1.0.0)
- Short description (one-line summary)
- Full description (multi-paragraph with bullets)
- Category, ratings, install count

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

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

smoothschedule/schedule/serializers.py

@@ -626,7 +626,7 @@ class PluginTemplateListSerializer(serializers.ModelSerializer):
     class Meta:
         model = PluginTemplate
         fields = [
-            'id', 'name', 'slug', 'short_description',
+            'id', 'name', 'slug', 'short_description', 'description',
             'visibility', 'category', 'tags',
             'author_name', 'version', 'license_type', 'is_approved',
             'install_count', 'rating_average', 'rating_count',