Add PDF processing and multi-format document conversion

Features added:
- PDF to image conversion with configurable DPI
- Multi-page PDF processing with OCR
- Export to Markdown, HTML, DOCX, and JSON formats
- Automatic image extraction from PDFs
- Formula and formatting preservation
- Real-time progress tracking for multi-page documents

Backend changes:
- New /api/process-pdf endpoint for PDF processing
- pdf_utils.py: PDF conversion and image extraction utilities
- format_converter.py: Document format conversion (MD, HTML, DOCX)
- Updated dependencies: PyMuPDF, img2pdf, python-docx, markdown

Frontend changes:
- File type toggle (Image OCR / PDF Processing)
- PDFProcessor component with format selection
- Updated ImageUpload to support both images and PDFs
- Progress bars for multi-page processing
- Download options for converted documents

Documentation:
- Updated README with PDF processing features
- Added API documentation for /api/process-pdf endpoint
- Added format conversion examples

backend/requirements.txt

@@ -11,3 +11,7 @@ pillow
 safetensors
 torch
 python-decouple>=3.8
+PyMuPDF>=1.23.0
+img2pdf>=0.5.0
+python-docx>=1.1.0
+markdown>=3.5.0