fix: correct OCR coordinate scaling by inferring dimensions from bbox

Critical Fix:
The previous implementation incorrectly calculated scale factors because
calculate_page_dimensions() was prioritizing source file dimensions over
OCR coordinate analysis, resulting in scale=1.0 when it should have been ~0.27.

Root Cause:
- PaddleOCR processes PDFs at high resolution (e.g., 2185x3500 pixels)
- OCR bbox coordinates are in this high-res space
- calculate_page_dimensions() was returning source PDF size (595x842) instead
- This caused scale_w=1.0, scale_h=1.0, placing all text out of bounds

Solution:
1. Rewrite calculate_page_dimensions() to:
   - Accept full ocr_data instead of just text_regions
   - Process both text_regions AND layout elements
   - Handle polygon bbox format [[x,y], ...] correctly
   - Infer OCR dimensions from max bbox coordinates FIRST
   - Only fallback to source file dimensions if inference fails

2. Separate OCR dimensions from target PDF dimensions:
   - ocr_width/height: Inferred from bbox (e.g., 2185x3280)
   - target_width/height: From source file (e.g., 595x842)
   - scale_w = target_width / ocr_width (e.g., 0.272)
   - scale_h = target_height / ocr_height (e.g., 0.257)

3. Add PyPDF2 support:
   - Extract dimensions from source PDF files
   - Required for getting target PDF size

Changes:
- backend/app/services/pdf_generator_service.py:
  - Fix calculate_page_dimensions() to infer from bbox first
  - Add PyPDF2 support in get_original_page_size()
  - Simplify scaling logic (removed ocr_dimensions dependency)
  - Update all drawing calls to use target_height instead of page_height

- requirements.txt:
  - Add PyPDF2>=3.0.0 for PDF dimension extraction

- backend/test_bbox_scaling.py:
  - Add comprehensive test for high-res OCR → A4 PDF scenario
  - Validates proper scale factor calculation (0.272 x 0.257)

Test Results:
✓ OCR dimensions correctly inferred: 2185.0 x 3280.0
✓ Target PDF dimensions extracted: 595.3 x 841.9
✓ Scale factors correct: X=0.272, Y=0.257

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

requirements.txt

@@ -22,6 +22,7 @@ opencv-python>=4.8.0
 weasyprint>=60.0
 markdown>=3.5.0
 reportlab>=4.0.0  # Layout-preserving PDF generation with precise coordinate control
+PyPDF2>=3.0.0  # Extract dimensions from source PDF files
 # Note: pandoc needs to be installed via brew (brew install pandoc)
 
 # ===== Data Export =====