worker/jobs/send_publish_notifications: Migrate to Emails::async_send()

[Turbo87]

less spawn_blocking(), more better!

[codecov]

Codecov Report

Attention: Patch coverage is 82.22222% with 8 lines in your changes missing coverage. Please review.

Project coverage is 89.17%. Comparing base (2f4ed8a) to head (504e751).
Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
src/worker/jobs/send_publish_notifications.rs	82.22%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9965      +/-   ##
==========================================
- Coverage   89.17%   89.17%   -0.01%     
==========================================
  Files         292      292              
  Lines       30380    30376       -4     
==========================================
- Hits        27092    27088       -4     
  Misses       3288     3288              

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