feat: /lendings api 명세 작성

[scarf005]

Summary

• resolves lendings API 명세 작성 #45

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 35 lines in your changes missing coverage. Please review.

Project coverage is 3.10%. Comparing base (a48a274) to head (a6c1571).
Report is 13 commits behind head on main.

Files with missing lines	Patch %	Lines
backend/src/lendings/lendings.controller.ts	0.00%	28 Missing ⚠️
backend/src/dto.ts	0.00%	7 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##            main     #51      +/-   ##
========================================
- Coverage   3.31%   3.10%   -0.22%     
========================================
  Files         34      36       +2     
  Lines        513     548      +35     
========================================
  Hits          17      17              
- Misses       496     531      +35     

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