Add admin API endpoints

[allenporter]

These are parallel to existing registration endpoints for admin use.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 89.65517% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.59%. Comparing base (fcf40bb) to head (363466e).

Files with missing lines	Patch %	Lines
supernote/server/routes/admin.py	87.50%	5 Missing ⚠️
supernote/server/services/user.py	93.75%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #53      +/-   ##
==========================================
+ Coverage   70.37%   70.59%   +0.21%     
==========================================
  Files          62       63       +1     
  Lines        6141     6193      +52     
==========================================
+ Hits         4322     4372      +50     
- Misses       1819     1821       +2     

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