✨(backend) serialize room pin code for frontend access

Add pin code to API response to enable frontend display of room access
codes. UI implementation will follow in upcoming commits.

src/backend/core/api/serializers.py

@@ -107,8 +107,8 @@ class RoomSerializer(serializers.ModelSerializer):
 
     class Meta:
         model = models.Room
-        fields = ["id", "name", "slug", "configuration", "access_level"]
-        read_only_fields = ["id", "slug"]
+        fields = ["id", "name", "slug", "configuration", "access_level", "pin_code"]
+        read_only_fields = ["id", "slug", "pin_code"]
 
     def to_representation(self, instance):
         """