fix(worker): batch flag comparison updates in ComputeComparison

[sentry]

This PR addresses an N+1 query performance issue in the ComputeComparisonTask.create_or_update_flag_comparisons method within apps/worker/tasks/compute_comparison.py.

Previously, the method would update existing CompareFlag rows and create new ones in a loop, leading to individual UPDATE statements and db_session.flush() calls for each new row. This resulted in an N+1 query pattern, as observed in Sentry traces.

The fix refactors the logic to:

• Collect all updates for existing CompareFlag objects into a list of dictionaries, ensuring uniform keys for efficient batching by SQLAlchemy.
• Collect all newly created CompareFlag objects into a separate list without immediate flushing.
• Remove the per-iteration db_session.flush() call from store_flag_comparison.
• Perform a single db_session.bulk_update_mappings for all existing rows and a single db_session.add_all for all new rows after the loop completes.
• Execute a single db_session.flush() at the end to persist all changes.
• Expire any CompareFlag objects already loaded in the session after the bulk update to ensure subsequent reads reflect the latest data.

This change significantly reduces the number of database queries, improving the performance of the ComputeComparison task.

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Fixes WORKER-PG3

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---

Note

Medium Risk
Changes how comparison flag rows are persisted (bulk ORM updates bypass the identity map until expire/flush), but behavior and fields should match prior logic in a background worker path.

Overview
Batches flag comparison DB writes in ComputeComparisonTask.create_or_update_flag_comparisons to remove per-flag UPDATE/flush N+1 behavior seen in worker traces.

Instead of mutating each existing CompareFlag in the loop and calling store_flag_comparison with immediate add/flush, the loop now collects new rows and update dicts (with a shared updated_at from get_utc_now()), then applies one bulk_update_mappings, expires loaded instances, add_all for inserts, and a single flush. store_flag_comparison only constructs and returns a CompareFlag using foreign-key IDs.

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ All tests successful. No failed tests found.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.65%. Comparing base (65af8ae) to head (b48860b).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2460   +/-   ##
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  Coverage   91.65%   91.65%           
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  Files        1341     1341           
  Lines       53342    53352   +10     
  Branches     1649     1649           
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+ Hits        48888    48898   +10     
  Misses       4133     4133           
  Partials      321      321           

Flag	Coverage Δ
workerintegration	58.45% <5.88%> (-0.04%)	⬇️
workerunit	90.56% <100.00%> (+<0.01%)	⬆️

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