fix: add unique constraint making country-currency combination

[uds5501]

Fixes fossasia/open-event-frontend#2169

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.

Short description of what this resolves:

This PR was created because #5657 is becoming increasingly tough to update on my local. So applied the appropriate changes and opening PR.

[uds5501]

@iamareebjamal @shreyanshdwivedi @mrsaicharan1 Please review

[codecov]

Codecov Report

Merging #6017 into development will increase coverage by <.01%.
The diff coverage is 100%.

@@               Coverage Diff               @@
##           development    #6017      +/-   ##
===============================================
+ Coverage        66.29%   66.29%   +<.01%     
===============================================
  Files              285      285              
  Lines            13997    13999       +2     
===============================================
+ Hits              9279     9281       +2     
  Misses            4718     4718

Impacted Files	Coverage Δ
app/models/ticket_fee.py	54.83% <100%> (+3.11%)	⬆️
app/api/schema/ticket_fees.py	100% <100%> (ø)	⬆️

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