feat(backend-services): initial invite codes

[jdvlio]

Initial implementation of invite code functionality. Invite codes are used to act as a gate for new users for the duration of the pilots.

[netlify[bot]]

✅ Deploy Preview for nautilus-wallet-staging canceled.

Name	Link
🔨 Latest commit	bb08bece131577df9ca503243fd41076de5dfec5
🔍 Latest deploy log	https://app.netlify.com/sites/nautilus-wallet-staging/deploys/643cdd9cb857e3000892bbd3

[codecov-commenter]

Codecov Report

Merging #548 (bb08bec) into main (c9cfcbc) will not change coverage. The diff coverage is n/a.

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@@           Coverage Diff           @@
##             main     #548   +/-   ##
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  Coverage   47.68%   47.68%           
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  Files         172      172           
  Lines        2760     2760           
  Branches      527      527           
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  Hits         1316     1316           
  Misses       1444     1444           

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