fix(worker): Optimize flake fetching in ProcessFlakesTask

[sentry]

The ProcessFlakesTask was experiencing slow DB queries due to the fetch_current_flakes function loading all historical flake records for a given repository. As the number of flakes accumulated over time, this led to an unbounded and inefficient query.

This change modifies fetch_current_flakes to filter for only active flakes by adding end_date__isnull=True to the query. This leverages the existing (repoid, end_date) index on the Flake model, significantly reducing the number of rows fetched and improving query performance for the ProcessFlakesTask.

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Fixes WORKER-Z1G

[codecov-notifications]

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 91.83%. Comparing base (c51397d) to head (32e7212).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1549   +/-   ##
=======================================
  Coverage   91.83%   91.83%           
=======================================
  Files        1326     1326           
  Lines       51047    51047           
  Branches     1631     1631           
=======================================
  Hits        46880    46880           
  Misses       3855     3855           
  Partials      312      312           

Flag	Coverage Δ
workerintegration	58.59% <ø> (ø)
workerunit	90.39% <ø> (ø)

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