fix: Prevent signup in backend when PREVENT_SIGNUP set to false

[khvn26]

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Changes

Fixes #4649.

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3 Skipped Deployments

Name	Status	Preview	Comments	Updated (UTC)
docs	⬜️ Ignored (Inspect)	Visit Preview		Sep 20, 2024 4:48pm
flagsmith-frontend-preview	⬜️ Ignored (Inspect)	Visit Preview		Sep 20, 2024 4:48pm
flagsmith-frontend-staging	⬜️ Ignored (Inspect)	Visit Preview		Sep 20, 2024 4:48pm

[github-actions]

Docker builds report

Image	Build Status	Security report
ghcr.io/flagsmith/flagsmith-e2e:pr-4650	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith-api-test:pr-4650	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith-frontend:pr-4650	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-api:pr-4650	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith:pr-4650	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-private-cloud:pr-4650	Finished ✅	Results ✅

[github-actions]

Uffizzi Preview deployment-56493 was deleted.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.17%. Comparing base (b74adbd) to head (1a79945).
Report is 6 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4650   +/-   ##
=======================================
  Coverage   97.17%   97.17%           
=======================================
  Files        1162     1162           
  Lines       40267    40273    +6     
=======================================
+ Hits        39130    39136    +6     
  Misses       1137     1137           

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