Use the sensitive source field in the serializer

Also correctly reference mature vs sensitive depending on what type of object is being handed into the serializer

api/api/serializers/media_serializers.py

@@ -516,6 +516,7 @@ class Meta:
 
     mature = serializers.BooleanField(
         help_text="Whether the media item is marked as mature",
+        source="sensitive",
     )
 
     # This should be promoted to a stable field alongside
@@ -540,9 +541,9 @@ def get_unstable__sensitivity(self, obj: Hit | AbstractMedia) -> list[str]:
         ):
             result.append(sensitivity.TEXT)
 
-        # ``obj.mature`` will either be `mature` from the ES document
-        # or the ``mature`` property on the Image or Audio model.
-        if obj.mature:
+        # ``obj.sensitive`` will either be `mature` from the ES document (see below)
+        # or the ``sensitive`` property on the Image or Audio model.
+        if obj.sensitive:
             # We do not currently have any documents marked `mature=true`
             # that were not marked so as a result of a confirmed user report.
             # This is despite the fact that the ingestion server _does_ copy
@@ -569,6 +570,16 @@ def get_unstable__sensitivity(self, obj: Hit | AbstractMedia) -> list[str]:
         return result
 
     def to_representation(self, *args, **kwargs):
+        # This serializer adapts both ES Hits *and* Media instances. Currently,
+        # ES has a `mature` field on it which represents if maturity was present on
+        # the record in the database. The attributes in the code have been renamed
+        # to `sensitive`, but for the time being this flag still exists on the ES index.
+        # In order to prevent failures in serialization (since the serializer is looking
+        # for the `sensitive` attribute), we rename it here.
+        obj = args[0]
+        if isinstance(obj, Hit):
+            obj.sensitive = obj.mature
+
         output = super().to_representation(*args, **kwargs)
 
         # Ensure lists are ``[]`` instead of ``None``