fix: Include platform plugins in marketplace and fix serializer

Changes:
1. Updated PluginTemplateViewSet.get_queryset() to include PLATFORM
   visibility plugins in marketplace view alongside approved PUBLIC plugins
2. Fixed PluginTemplateListSerializer read_only_fields from string '__all__'
   to proper list reference

Platform plugins are now visible in the marketplace API:
- 6 platform-created plugins seeded via seed_platform_plugins command
- Categories: EMAIL, REPORTS, CUSTOMER, BOOKING
- All marked as visibility=PLATFORM, is_approved=True

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

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

smoothschedule/schedule/views.py

@@ -642,10 +642,11 @@ class PluginTemplateViewSet(viewsets.ModelViewSet):
         view_mode = self.request.query_params.get('view', 'marketplace')
 
         if view_mode == 'marketplace':
-            # Public marketplace - only approved public templates
+            # Public marketplace - platform official + approved public templates
+            from django.db.models import Q
             queryset = queryset.filter(
-                visibility=PluginTemplate.Visibility.PUBLIC,
-                is_approved=True
+                Q(visibility=PluginTemplate.Visibility.PLATFORM) |
+                Q(visibility=PluginTemplate.Visibility.PUBLIC, is_approved=True)
             )
         elif view_mode == 'my_plugins':
             # User's own templates