🐛(backend) fix issues with conversion microservice integration

Minor adjustments were needed after working in parallel on two PRs.
The microservice now accepts an API key without requiring it as a Bearer token.

A mistake in reading the microservice response was corrected after refactoring
the serializer to delegate logic to the converter microservice.

src/backend/impress/settings.py

@@ -524,6 +524,11 @@ class Base(Configuration):
         environ_name="CONVERSION_API_TIMEOUT",
         environ_prefix=None,
     )
+    CONVERSION_API_SECURE = values.Value(
+        default=False,
+        environ_name="CONVERSION_API_SECURE",
+        environ_prefix=None,
+    )
 
     # Logging
     # We want to make it easy to log to console but by default we log production