500: on temp user creation

[smadbe]

Investigate this bug:

{"http_method":"POST",
"http_proto":"HTTP/1.1",
"http_scheme":"http",
"level":"error",
"msg":"unexpected error: Error 1062: Duplicate entry 'tmp-22576773' for key 'users.login', stack trace: goroutine 1 [running]:\nruntime/debug.Stack()\n\t/usr/local/go/src/runtime/debug/stack.go:24 +0x65\ngithub.com/France-ioi/AlgoreaBackend/v2/app/service.AppHandler.ServeHTTP.func1()\...","time":"2024-09-20T06:28:18Z","ts":"Fri, 20 Sep 2024 06:28:18 UTC",
"type":"web",
"uri":"http://parcours.algorea.org/api/auth/token?cookie_same_site=1\u0026cookie_secure=1\u0026create_temp_user_if_not_authorized=1\u0026default_language=fr\u0026use_cookie=1",
"user_agent":"Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/128.0.0.0 Safari/537.36"}

There was already a "tmp-22576773" created on 6/09 (so 14 days earlier)... so may it be a pure collision? are collisions handled?