Uploading 100k Entities, frontend reports 504 and stays on preview screen, upload has actually succeeded

#### Problem description
I uploaded https://drive.google.com/file/d/1y2Z9ZwHcX60FRW5F2vxbolooj3F6-bgY/view?usp=drive_link which has 100k Entities. After a minute, I saw a 504 at the top of the modal with the append button. I exited the modal and saw my Entities were successfully created.

#### URL of the page
https://staging.getodk.cloud/#/projects/93/entity-lists/entities_100k/entities

#### Expected behavior
~In the case of a 504, I think we should close the modal if possible. Because there's no duplicate detection, a user is at high risk of uploading the same Entities twice and then they're stuck with them.~

Alternately, could the server send back something to say it's still working on it?

#### Central version shown in version.txt
```
versions:
e49518adb84f88d7bc6c3626fc77584dfc935435 (v2024.1.0-6-ge49518a)
+988780e2d439894ef6e5af15692a8916c7c8d8e5 client (v2024.1.0-23-g988780e2)
+fb96aa3333d3442ea43365755766262f21de3969 server (v2024.1.0-23-gfb96aa33)
```

#### Browser version

#### Around when did you see the problem (in UTC)?

#### Other notes (if any)

No files, tests, or entry points are named in the issue. Reproduce an upload of 100k Entities from the linked staging page and trace the 504 response and modal state; done means the completed upload does not leave the user on the preview screen or encourage a duplicate upload.

Written by the indexing model from the issue text.