✨(frontend) serialize recording key for frontend download links

Add recording key to serialized API response to enable frontend to generate
proper download links without additional backend calls. Simplifies media
access workflow across the application.

src/backend/core/api/serializers.py

@@ -156,7 +156,7 @@ class RecordingSerializer(serializers.ModelSerializer):
 
     class Meta:
         model = models.Recording
-        fields = ["id", "room", "created_at", "updated_at", "status", "mode"]
+        fields = ["id", "room", "created_at", "updated_at", "status", "mode", "key"]
         read_only_fields = fields