Fix: Add Validation for Environment Variables in config.py

[kunalbhardwaj2006]

This PR improves configuration safety by validating environment variables in config.py.

Changes:

• Added helper functions to safely parse integer and float environment variables.
• Added validation for IMAGE_FORMAT to ensure only 'png' or 'webp' are allowed.
• Added range validation for IMAGE_QUALITY (1–100).
• Replaced assert with explicit validation for CLASSIFICATION_BATCH_SIZE.
• Added validation for PDF_ZOOM_LEVEL to ensure positive values.

These improvements prevent misconfiguration and provide clearer error messages when environment variables contain invalid values.

[kunalbhardwaj2006]

Hi maintainers 👋

I noticed that some environment variables in config.py were being parsed directly using int() and assert, which could lead to unclear errors if invalid values are provided.

This PR adds validation and safe parsing helpers to improve configuration robustness and provide clearer error messages.

Happy to adjust if there is a preferred configuration pattern in the project.