Feat: Add WebP Image Format Support

[skyforge-glitch]

Describe the feature

Summary

PictoPy currently supports PNG, JPG, and JPEG image formats. WebP images are ignored during uploads due to backend and frontend validation, even though Pillow is capable of decoding WebP images. This issue proposes adding official WebP support.

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Problem

• .webp images are silently skipped during file or folder uploads
• No error or feedback is shown to the user
• Backend image pipeline does not crash, indicating this is a validation-level limitation rather than a decoding issue

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Proposed Solution

1. Verify Pillow compatibility for WebP decoding in the backend environment
2. Extend backend validation to allow:
   • .webp file extension
   • image/webp MIME type
3. Update frontend validation:
   • Allow .webp in file picker and drag-and-drop
   • Update supported formats text if applicable
4. Ensure existing image processing, tagging, and clustering pipelines remain unchanged

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Scope

• No changes to ML, tagging, or clustering logic
• No database schema changes
• Backend and frontend validation updates only

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Benefits

• Adds support for a modern and widely used image format
• Improves compatibility with images from browsers, mobile devices, and cloud exports
• Non-breaking enhancement with minimal risk

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Acceptance Criteria

• WebP images can be uploaded via file picker and folder upload
• WebP images are visible and processed like other supported formats
• Existing PNG/JPG/JPEG functionality remains unaffected

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Record

• I agree to follow this project's Code of Conduct
• I want to work on this issue

[github-actions]

👋 Thanks for opening this issue! The maintainers will review it shortly. Till then, do not open any PR. This is a strict rule we like to follow around here :)

[skyforge-glitch]

@rahulharpal1603 As discussed, requesting assignment for this issue.