feat: Add inline image support to document templates

[christoph2806]

🖼️ Image Support Implementation

This PR adds comprehensive inline image support to the Document Template Processing Service, enabling embedding of PNG images directly in Word document templates.

✨ Key Features

• 🎯 New API Endpoint:
• 📱 React-Friendly: Base64 image encoding in JSON payloads
• 📏 Flexible Sizing: Support for both millimeters (print) and pixels (web)
• 🔧 Template Integration: Simple Jinja2 syntax like {{ company_logo }}
• 🛡️ Error Handling: Comprehensive image processing error management
• 🔄 Backward Compatible: Original endpoint remains unchanged

🚀 Implementation Highlights

• Leverages docxtpl's InlineImage functionality
• Pydantic models for structured request validation
• Automatic DPI conversion (96px = 25.4mm)
• Temporary file management with proper cleanup
• Comprehensive test suite with automated validation

📋 API Example

{
  "template_data": {
    "customer_name": "John Doe",
    "logo": "{{company_logo}}"
  },
  "images": {
    "company_logo": {
      "data": "base64_encoded_png_data",
      "width_mm": 50,
      "height_mm": 20
    }
  }
}

📚 Documentation Added

• Image Support Guide - Complete implementation guide
• React integration examples with hooks
• cURL examples and troubleshooting
• Best practices for image optimization

✅ Testing

• ✅ Image processing with Base64 encoding
• ✅ PDF generation with embedded images
• ✅ Multiple image support
• ✅ Backward compatibility maintained
• ✅ Comprehensive error handling validated

🔧 Files Changed

• main.py - Core image processing implementation
• docs/image-support.md - New comprehensive documentation
• test_image_support.py - Automated test suite
• requirements-test.txt - Added Pillow for testing
• Updated navigation in README.md, docs/README.md, index.md

Ready for review and testing! 🚀