🐛(backend) Forbid restoring a non-deleted document

Catch RuntimeError raised by Document.restore() and translate it into a
DRF ValidationError so callers get a 400 instead of a 500, when trying
to restore a non-deleted document.
    
Signed-off-by: Mohamed El Amine BOUKERFA <boukerfa.ma@gmail.com>

src/backend/core/api/viewsets.py

@@ -996,7 +996,10 @@ class DocumentViewSet(
         Restore a soft-deleted document if it was deleted less than x days ago.
         """
         document = self.get_object()
-        document.restore()
+        try:
+            document.restore()
+        except RuntimeError as err:
+            raise drf.exceptions.ValidationError({"detail": str(err)}) from err
 
         return drf_response.Response(
             {"detail": "Document has been successfully restored."},