worker/jobs/daily_db_maintenance: Remove spawn_blocking() usage

[Turbo87]

We can use diesel-async instead 🎉

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 7 lines in your changes missing coverage. Please review.

Project coverage is 88.70%. Comparing base (59303c8) to head (b195c01).
Report is 8 commits behind head on main.

Files with missing lines	Patch %	Lines
src/worker/jobs/daily_db_maintenance.rs	0.00%	7 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #9653   +/-   ##
=======================================
  Coverage   88.69%   88.70%           
=======================================
  Files         288      288           
  Lines       29371    29368    -3     
=======================================
  Hits        26052    26052           
+ Misses       3319     3316    -3     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.