User game export API has undocumented max value defaulting to 5000

[Siderite]

Exact URL of where the bug happened

https://lichess.org/@/KamranShirazi/download

Steps to reproduce the bug

1. Go to the URL above
2. Export all games without changing anything

What did you expect to happen?

Get all the games of the user

What happened instead?

Got exactly 5000 games.

Operating system

Any

Browser and version (or alternate access method)

Any

Additional information

I think this was introduced in Commit 53eac1d

The problem is not that it returns 5000 games, but that it's completely unexpected. Neither the export page, nor the API documentation mention this. In fact, the API reference (https://lichess.org/api#tag/Games/operation/apiGamesUser) says "Leave empty to download all games".