fix(backend): resolve OCRService strict base64 line break rejection and CPU blocking DoS

[harshitanagpal05]

Summary

We have successfully resolved the base64 input validation functional bug and CPU exhaustion performance vulnerability in the OCR service.

Proposed Changes

• Optimized Regex Lookup: Replaced the synchronous, CPU-blocking character loop in OCRService.extract_text with a compiled C regular expression lookup to allow standard whitespaces and line wraps (\n, \r).
• MIME Base64 Normalization: Safely strips internal line breaks and whitespace before computing bounds and decoding, resolving compatibility across browser and mobile encoders.
• Asynchronous Unit Tests Refactoring: Rewrote test_ocr_service.py to be fully asynchronous using pytest-anyio.
• Pillow Mocking: Patched PIL.Image.open inside unit tests to return a mock Image with custom bounds, preventing format exceptions from failing tests.
  closes bug(backend): strict all() base64 checking in OCRService rejects valid base64 strings with newlines and causes CPU-blocking DoS #793

Verification Results

All 8 async unit tests pass cleanly:

backend\tests\test_ocr_service.py ........                               [100%]
======================== 8 passed in 60.26s (0:01:00) =========================

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[ritesh-1918]

Superb implementation, @harshitanagpal05! I've successfully resolved all conflicts in your PR and queued it for merging into gssoc.

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