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

### Describe the feature

## Is there an existing issue for this?

- [x] 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
```python
allowed_extensions = {".jpg", ".jpeg", ".png"}

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- [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?