[bug] Double-post on SIWFv2 login callback

[JoeCap08055]

Description

When SAT frontend receives a login/signup callback from Frequency Access, the SAT frontend appears to be sending the verification payload to the backend (and thence through to the Gateway Account Service) twice

Diagnostics/Steps to Reproduce

1. Navigate to SAT frontend
2. "Signup/Login with Frequency v2"
3. Go through signup/login flow, receive verification email
4. Watch:
   • account-service-api logs
   • Browser dev tools "Network" tab in SAT frontend window
5. Enter verification code from email

Expected Results

• SAT frontend /login/callback route received ONCE
• SAT frontend makes ONE call to SAT backend /auth/login/v2/siwf/verify
• account-service-api receives ONE request at /v2/accounts/siwf with an authorizationCode from the callback

Actual Results:

• SAT frontend /login/callback route received ONCE (seen in Network tab)
• SAT frontend makes 2 calls to SAT backend /auth/login/v2/siwf/verify (seen in Network tab)
• account-service-api receives 2 identical requests at /v2/accounts/siwf with the same authorizationCode (seen in logs)