docs: add changelog entry for issue #7 fix

[atsyplenkov]

Summary

• add an Unreleased changelog entry for the OCR concurrency/rate-limit fix
• reference closed issue Rate limit #7 in changelog

Test Plan

• docs-only change

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.26%. Comparing base (e8c8d21) to head (31a2af2).
⚠️ Report is 2 commits behind head on main.
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Additional details and impacted files

@@           Coverage Diff           @@
##             main      #10   +/-   ##
=======================================
  Coverage   85.26%   85.26%           
=======================================
  Files           9        9           
  Lines        1751     1751           
=======================================
  Hits         1493     1493           
  Misses        258      258           

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