fix: unique ZIP folders for same-named batch PDFs

[nyxsky404]

Pull Request

Issue Reference

Closes #606

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Summary

• allocate unique ZIP folder names when batch inputs share a filename
• keep first folder as the base name and suffix later collisions
• add unit coverage for the folder-name allocator

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Screenshots (if applicable)

NA

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Checklist

• My code follows the project's coding conventions
• I have tested all impacted features
• I have updated or added necessary documentation

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Related Issues / PRs

NA

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Open Source Program Participation

Program Name: GSSoC

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Additional Notes

Focused Vitest checks, typecheck, and production build pass.

[vercel]

@nyxsky404 is attempting to deploy a commit to the Durgeshwar's projects Team on Vercel.

A member of the Team first needs to authorize it.

[nyxsky404]

@Durgeshwar-AI Merge conflicts with main are resolved (README kept both the unique batch ZIP-folder note and upstream’s same-name merge wording).

The remaining backend CI failure (test_add_watermark_* unpacking error in blueprints/watermark.py) is already present on upstream main, and is unrelated to this PR. The Vercel check is failing on team authorization, not on this PR’s code.