fix: PLT-796: Prevent deadlocks with batch updates

[triklozoid]

When multiple users are simultaneously working on projects with a large number of tasks, PostgreSQL deadlocks were occurring during massive tasks update operations. These deadlocks happen when the _rearrange_overlap_cohort() method tries to update thousands of task records in a single database
transaction while other users are also modifying tasks in the same project.

• Split large UPDATE operations into batches (500 records)
• Added retry logic with exponential backoff (up to 3 attempts)
• Updated _rearrange_overlap_cohort() to use batch updates

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[codecov]

Codecov Report

Attention: Patch coverage is 96.42857% with 1 line in your changes missing coverage. Please review.

Project coverage is 78.85%. Comparing base (44b34f5) to head (450fbe0).
Report is 9 commits behind head on develop.

✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
label_studio/projects/models.py	96.42%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #7752      +/-   ##
===========================================
+ Coverage    78.83%   78.85%   +0.02%     
===========================================
  Files          205      205              
  Lines        16336    16358      +22     
===========================================
+ Hits         12878    12899      +21     
- Misses        3458     3459       +1     

Flag	Coverage Δ
pytests	78.85% <96.42%> (+0.02%)	⬆️

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