worker/jobs/sync_admins: Reduce spawn_blocking() usage

[Turbo87]

Unfortunately we can't completely avoid it yet, since our email sending code is still synchronous, but we'll get there eventually...

(best reviewed with "hide whitespace changes" 😉)

[codecov]

Codecov Report

Attention: Patch coverage is 97.56098% with 2 lines in your changes missing coverage. Please review.

Project coverage is 88.69%. Comparing base (59303c8) to head (26b14b1).
Report is 9 commits behind head on main.

Files with missing lines	Patch %	Lines
src/worker/jobs/sync_admins.rs	97.56%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9654      +/-   ##
==========================================
- Coverage   88.69%   88.69%   -0.01%     
==========================================
  Files         288      288              
  Lines       29371    29374       +3     
==========================================
+ Hits        26052    26054       +2     
- Misses       3319     3320       +1     

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