controllers/user/session: Remove spawn_blocking() usage

[Turbo87]

less blocking worker threads, more async :)

[codecov]

Codecov Report

Attention: Patch coverage is 74.43609% with 34 lines in your changes missing coverage. Please review.

Project coverage is 89.22%. Comparing base (b47ff50) to head (09bf303).
Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
src/controllers/user/session.rs	32.00%	34 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9972      +/-   ##
==========================================
+ Coverage   89.21%   89.22%   +0.01%     
==========================================
  Files         293      293              
  Lines       30390    30394       +4     
==========================================
+ Hits        27113    27120       +7     
+ Misses       3277     3274       -3     

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