fix: Add public payment gateway key in settings

[divs4debu]

Fixes #7187

Short description of what this resolves:

Changes proposed in this pull request:

Checklist

• I have read the Contribution & Best practices Guide and my PR follows them.
• My branch is up-to-date with the Upstream development branch.
• The unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

• All the functions created/modified in this PR contain relevant docstrings.

[houndci-bot]

Black would make changes.

[codecov]

Codecov Report

Merging #7197 into development will increase coverage by 0.14%.
The diff coverage is 100.00%.

@@               Coverage Diff               @@
##           development    #7197      +/-   ##
===============================================
+ Coverage        62.87%   63.02%   +0.14%     
===============================================
  Files              259      259              
  Lines            12992    13013      +21     
===============================================
+ Hits              8169     8201      +32     
+ Misses            4823     4812      -11     

Impacted Files	Coverage Δ
app/api/schema/settings.py	100.00% <100.00%> (ø)
app/api/helpers/mail.py	25.64% <0.00%> (-1.72%)	⬇️
app/api/helpers/permission_manager.py	58.39% <0.00%> (+0.69%)	⬆️
app/api/sessions.py	58.75% <0.00%> (+2.04%)	⬆️
app/models/setting.py	93.96% <0.00%> (+7.75%)	⬆️
app/api/settings.py	72.72% <0.00%> (+11.36%)	⬆️

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