continue in bg doesn't work on rageshake

it always fails with a 400 or other error if i hit continue in bg to upload logs. see my most recent rageshake for an example

Start by reproducing the failure in the iOS app: submit a rageshake and choose “continue in background,” then inspect the background log-upload flow and the resulting 400 response. Done means the rageshake upload completes successfully after choosing that option.

Written by the indexing model from the issue text.