refactor(worker): Switch to asyncio for get_repo_files

[nellh]

Migrate get_repo_files and get_repo_urls to asyncio. This avoids blocking the uvicorn process for expensive trees that can sometimes result in a health check exceeding the timeout and restarting the worker unnecessarily.

[codecov]

Codecov Report

❌ Patch coverage is 87.50000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 42.84%. Comparing base (d86625c) to head (66b8292).
⚠️ Report is 14 commits behind head on master.

Files with missing lines	Patch %	Lines
services/datalad/datalad_service/common/annex.py	85.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3730      +/-   ##
==========================================
- Coverage   42.85%   42.84%   -0.01%     
==========================================
  Files         640      640              
  Lines       34364    34379      +15     
  Branches     1548     1548              
==========================================
+ Hits        14725    14731       +6     
- Misses      19500    19509       +9     
  Partials      139      139              

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