Add explicit PH event for verification

[joepavitt]

Description

• If a user presses <enter> on the verification code, it's very difficult to reliably track that in PH as we have no unique form identifier, or unique URL for the verification form.
• The other challenge with relying on the autocapture events is that even if an incorrect code is submitted, we still register the click as a successful "verify"
• A cleaner way to handle this is have an explicit event called when the API responds successfully, which this PR adds

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.17%. Comparing base (ef94197) to head (13f6190).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4344   +/-   ##
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  Coverage   78.17%   78.17%           
=======================================
  Files         292      292           
  Lines       13430    13430           
  Branches     3010     3010           
=======================================
  Hits        10499    10499           
  Misses       2931     2931           

Flag	Coverage Δ
backend	78.17% <ø> (ø)

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