Feat: Add support for additional image formats (WebP, BMP, TIFF, GIF)

[Rudra-Tiwari-codes]

Describe the feature

Is there an existing issue for this?

• I have searched the existing issues

Feature Description

Currently, PictoPy only supports .jpg, .jpeg, and .png image formats. Modern photo libraries often contain images in other popular formats that should be supported.

Current Behavior

File: backend/app/utils/images.py - image_util_is_valid_image() function

allowed_extensions = {".jpg", ".jpeg", ".png"}

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- [x] I agree to follow this project's Code of Conduct
- [x] I want to work on this issue

### Checklist before Submitting.

- [x] Have you updated docs for it?.
- [x] Have you added unit tests?.
- [x] Have you made sure unit tests pass?
- [x] Have you made sure code formatting is correct?
- [x] Does it contain any style related issues?

[Rudra-Tiwari-codes]

Hi maintainers 👋

I’m interested in this issue and I’ve started working on it.
I’ll raise a PR once the changes are ready. Please let me know if there’s anything specific you’d like me to follow.

Thanks!