feat: implement core dual-track processing infrastructure

Added foundation for dual-track document processing:

1. UnifiedDocument Model (backend/app/models/unified_document.py)
   - Common output format for both OCR and direct extraction
   - Comprehensive element types (23+ types from PP-StructureV3)
   - BoundingBox, StyleInfo, TableData structures
   - Backward compatibility with legacy format

2. DocumentTypeDetector Service (backend/app/services/document_type_detector.py)
   - Intelligent document type detection using python-magic
   - PDF editability analysis using PyMuPDF
   - Processing track recommendation with confidence scores
   - Support for PDF, images, Office docs, and text files

3. DirectExtractionEngine Service (backend/app/services/direct_extraction_engine.py)
   - Fast extraction from editable PDFs using PyMuPDF
   - Preserves fonts, colors, and exact positioning
   - Native and positional table detection
   - Image extraction with coordinates
   - Hyperlink and metadata extraction

4. Dependencies
   - Added PyMuPDF>=1.23.0 for PDF extraction
   - Added pdfplumber>=0.10.0 as fallback
   - Added python-magic-bin>=0.4.14 for file detection

Next: Integrate with OCR service for complete dual-track processing

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

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

requirements.txt

@@ -25,6 +25,11 @@ reportlab>=4.0.0  # Layout-preserving PDF generation with precise coordinate con
 PyPDF2>=3.0.0  # Extract dimensions from source PDF files
 # Note: pandoc needs to be installed via brew (brew install pandoc)
 
+# ===== Direct PDF Extraction (Dual-track Processing) =====
+PyMuPDF>=1.23.0  # Primary library for editable PDF text/structure extraction
+pdfplumber>=0.10.0  # Fallback for table extraction and validation
+python-magic-bin>=0.4.14  # Windows-compatible file type detection
+
 # ===== Data Export =====
 pandas>=2.1.0
 openpyxl>=3.1.0  # Excel support