fix(api): delegate concurrent deletion deadlocks to TaskBackoffError

[SahilJat]

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Changes

Closes #6878

Please describe.
1.Wrapped the deletion queries in environments/tasks.py and features/tasks.py with specific exception handlers for OperationalError, IntegrityError, and TransactionManagementError.
2. Delegated the retry mechanism by raising TaskBackoffError.
3. This allows the first worker to hold the lock and complete its softdelete cascade, while the colliding worker gracefully yields, backs off, and safely retries without polluting Sentry with unhandled DB exceptions.

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Please describe.

[claude]

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[codecov]

Codecov Report

❌ Patch coverage is 71.42857% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 98.23%. Comparing base (67fa5fd) to head (e7cf0a6).
⚠️ Report is 8 commits behind head on main.

Files with missing lines	Patch %	Lines
api/environments/tasks.py	71.42%	2 Missing ⚠️
api/features/tasks.py	71.42%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6995      +/-   ##
==========================================
- Coverage   98.31%   98.23%   -0.09%     
==========================================
  Files        1335     1335              
  Lines       49784    49669     -115     
==========================================
- Hits        48947    48793     -154     
- Misses        837      876      +39     

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