[bugfix] Fix POST to create account endpoint

[tsmethurst]

Description

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Lil fix + new test for creating an account by POSTing to the accounts endpoint.

Following command on the testrig:

curl -H 'content-type: application/json' -H 'authorization: Bearer ZTK1MWMWZDGTMGMXOS0ZY2UXLWI5ZWETMWEZYZZIYTLHMZI4' -d '{"username":"xk063c2xa3insqw3n4nj_sb_persona","email":"[email protected]","password":"20 char long mix of upper lower special and digits","reason":"a long enough explanation of why I am doing api calls","agreement":true,"locale":"en-US"}' localhost:8080/api/v1/accounts

Now returns expected response with default scope (it's a bit dorky that it's an empty string but it's got nothing to do with this PR at least):

{
  "access_token": "NDU5MZNLM2ITYJHLYY0ZYTIXLTG5NTCTNTEYOTJKYZK1ZGFM",
  "token_type": "Bearer",
  "scope": "",
  "created_at": 1739103202
}

closes #3733
closes #3612

Checklist

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