test(server): reproduce user ID reuse and file leakage after unregister

[allenporter]

Expands test_user_unregister in tests/server/routes/test_auth.py to test the full lifecycle: creating files, unregistering, creating a subsequent user, and querying files.

Currently documents and tests the existing behavior with a TODO referencing #240 where user ID recycling and file isolation will be resolved.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.91%. Comparing base (ccc1999) to head (a07c37a).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #241      +/-   ##
==========================================
- Coverage   90.95%   90.91%   -0.04%     
==========================================
  Files         113      113              
  Lines       11749    11749              
==========================================
- Hits        10686    10682       -4     
- Misses       1063     1067       +4     

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