Refactor app.py and improve model loading and file uploading

[Shobhit141141]

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[Shobhit141141]

This pull request makes several positive changes to the app.py file, improving both the structure and functionality of the application. Here's my analysis:

Positive Changes:

• Model Loading: The code now gracefully handles the case where the model file (best_model.pkl) is not found. This is important for preventing crashes and providing a clear error message to the user.
• Custom Category Order: The introduction of CUSTOM_CATEGORY_ORDER is a clever way to control the display order of model output labels. This allows for improved user experience by presenting results in a more logical and intuitive manner.
• File Upload Folder Creation: The code now automatically creates the upload folder (uploads) if it doesn't exist. This prevents potential errors and ensures a smooth file upload process.
• Route Documentation: The use of comments to describe the purpose of the '/' route (@app.route('/')) is a good practice for better code clarity and maintainability.
• Error Handling in Uploader: The uploader() function now includes a try-except block to catch potential exceptions during the file processing. This provides more robust error handling, preventing the application from crashing and providing helpful error messages to the user.
• Code Formatting and Comments: The use of whitespace and comments throughout the code contributes to better readability and understanding.

Areas for Improvement:

• Model Load Error Handling: While the code now handles the case where the model file is not found, it might be beneficial to consider more robust error handling. For example, instead of simply raising a FileNotFoundError, you could provide a more user-friendly message on the frontend (perhaps via a flash message or a dedicated error page).
• File Upload Validation: Currently, there's no validation for the uploaded files. Adding validation to ensure that only PDF files are allowed would prevent unexpected issues. This could be done by checking the file extension or using the mimetypes library.
• Code Documentation: While the comments are helpful, adding more detailed documentation, potentially using docstrings, would further improve the code's clarity and maintainability. This would be particularly beneficial for larger projects.

Overall, I believe this pull request is a good step towards improving the app's stability and functionality. The changes enhance error handling, organization, and user experience. By addressing the suggested areas for improvement, the code could be further strengthened and made more robust.