[DOP-21799] Refactor celery initialization

[IlyasDevelopment]

Change Summary

• Moved Celery instances to separate backend, worker and scheduler modules
• Removed unnecessary variable definitions from docs/conf.py

Checklist

• Commit message and PR title is comprehensive
• Keep the change as small as possible
• Unit and integration tests for the changes exist
• Tests pass on CI and coverage does not decrease
• Documentation reflects the changes where applicable
• docs/changelog/next_release/<pull request or issue id>.<change type>.rst file added describing change
  (see CONTRIBUTING.rst for details.)
• My PR is ready to review.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 0.00%. Comparing base (60ab590) to head (ad54a47).
Report is 1 commits behind head on develop.

Additional details and impacted files

@@      Coverage Diff       @@
##   develop   #165   +/-   ##
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