test session expiry behaviour, use session cookies

[hillalex]

Convert the cookie to a session cookie (rather than fixed expiry). Test the behaviour that cleans up the file uploads when a session becomes inactive.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.51%. Comparing base (92fbc2e) to head (c53f343).
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main       #4      +/-   ##
==========================================
- Coverage   90.55%   90.51%   -0.05%     
==========================================
  Files           4        4              
  Lines         233      232       -1     
==========================================
- Hits          211      210       -1     
  Misses         22       22              

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