🚨(backend) handle new checks introduced in Pylint v3.2.0

Pylint 3.2.0 introduced a new check `possibly-used-before-assignment`, which
ensures variables are defined regardless of conditional statements.

Some if/else branches were missing defaults. These have been fixed.

src/backend/core/api/__init__.py

@@ -23,6 +23,8 @@ def exception_handler(exc, context):
             detail = exc.message
         elif hasattr(exc, "messages"):
             detail = exc.messages
+        else:
+            detail = ""
 
         exc = drf_exceptions.ValidationError(detail=detail)