backend_design.txt

The backend will be a Python Flask server with the following key components:

1. Flask app setup and routes:
   - POST /upload_image: Handle image upload
   - POST /generate_invoice: Generate invoice based on user input
   - GET /download_pdf: Serve generated PDF for download

2. Image handling:
   - Use Pillow library to process and store uploaded images

3. Invoice generation:
   - Create a function to generate invoice using user input (items, prices)
   - Use ReportLab library to create PDF invoices

4. Data storage:
   - Use SQLite database to store invoice data and image references

5. Dependencies:
   - Flask
   - Pillow
   - ReportLab
   - SQLite3

Key functions:
- upload_image(): Save uploaded image and return a reference ID
- generate_invoice(): Create invoice using user input and image reference
- create_pdf(): Use ReportLab to generate PDF invoice
- get_pdf(): Retrieve generated PDF for download

The server will handle image uploads, store invoice data, generate PDFs, and serve the frontend HTML page.