backend: Add death loop detection

[tianjing-li]

• Imports Death loop from eno

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 84.94624% with 14 lines in your changes missing coverage. Please review.

Project coverage is 79.97%. Comparing base (f90aae1) to head (c12ee8f).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
src/backend/tests/unit/services/test_chat.py	80.00%	10 Missing ⚠️
src/backend/chat/custom/custom.py	42.85%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #815      +/-   ##
==========================================
+ Coverage   79.92%   79.97%   +0.04%     
==========================================
  Files         242      243       +1     
  Lines       10305    10395      +90     
==========================================
+ Hits         8236     8313      +77     
- Misses       2069     2082      +13     

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[EugeneLightsOn]

Looks good to me. We need to re-sync it again after the Eno experiments are done.