fix: reject oversized PDFs in reorder export

[nyxsky404]

Pull Request

Issue Reference

Closes #607

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Summary

• reject reorder uploads over 50 pages instead of exporting a truncated PDF
• keep export disabled when the page-limit warning is active
• add regression coverage for truncated 51-page preview detection

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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 This PR is mergeable (no conflicts with main).

The failing backend CI (test_add_watermark_text_success / test_add_watermark_image_success — ValueError: not enough values to unpack in blueprints/watermark.py) is already present on upstream main, and is unrelated to this PR’s reorder size-limit changes. The Vercel check is failing on team authorization, not on this PR’s code.