feat: add OCR-specific concurrency control for batch processing

[atsyplenkov]

Summary

• add --ocr-workers (default 2) and keep --workers default 32
• cap OCR API concurrency with an async limiter while preserving broader batch parallelism
• improve HTTP 429 OCR failure messaging with actionable guidance and update docs/tests

Closes #7.

Test Plan

• cargo fmt --all -- --check
• cargo clippy --all-targets --all-features
• cargo test
• cargo test --features internal-testing --test core_internal

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 92.48120% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.26%. Comparing base (b9576c7) to head (e100965).

Files with missing lines	Patch %	Lines
src/core.rs	89.83%	6 Missing ⚠️
src/core/ocr.rs	91.30%	4 Missing ⚠️
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Additional details and impacted files

@@            Coverage Diff             @@
##             main       #8      +/-   ##
==========================================
+ Coverage   83.97%   85.26%   +1.28%     
==========================================
  Files           9        9              
  Lines        1629     1751     +122     
==========================================
+ Hits         1368     1493     +125     
+ Misses        261      258       -3     

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