[fix][backend] readme security note

[lsy357]

What type of PR is this?

fix

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[codecov]

Codecov Report

❌ Patch coverage is 40.00000% with 3 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
backend/infra/middleware/session/session.go	40.00%	1 Missing and 2 partials ⚠️

❌ Your patch status has failed because the patch coverage (40.00%) is below the target coverage (80.00%). You can increase the patch coverage or adjust the target coverage.

@@           Coverage Diff           @@
##             main     #479   +/-   ##
=======================================
  Coverage   75.45%   75.45%           
=======================================
  Files         633      633           
  Lines       67793    67798    +5     
=======================================
+ Hits        51153    51157    +4     
  Misses      13391    13391           
- Partials     3249     3250    +1     

Flag	Coverage Δ
unittests	75.45% <40.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
backend/infra/middleware/session/session.go	34.21% <40.00%> (+0.87%)	⬆️

... and 1 file with indirect coverage changes

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