Property-based test that AES envelope survives round-trip across all evidence sizes

[kilodesodiq-arch]

Problem Statement. UploadChunk and UploadSession schemas exist. There is no
apparent integration test asserting that arbitrary-size uploads (1 KB to 100 MB)
preserves checksum equality.

Technical Context.
app/backend/test/.

Expected Outcome. A property-based test (fast-check or jqwik) iterates over
chunks of random size and verifies checksum round-trip.

Acceptance Criteria.

• 1000 random sequences in CI under 30 s.

Files or modules likely to be affected.
new app/backend/test/upload-roundtrip.spec.ts,
package.json.

Difficulty. Medium
Estimated effort. S

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Backlog item #67 from `docs/maintainer-issue-backlog.md.

[yunus-dev-codecrafter]

Hi! I've carefully reviewed the entire task description and fully understand the requirements and expected outcome.
I have experience working on similar tasks and I'm confident I can complete this within about an hour. I'd be happy to take ownership of this issue and submit a clean, well-tested solution promptly.
If it's available, please assign it to me. Thank you!

[AugistineCreates]

I would like to work on this issue because i am confident i can resolve the issue, having worked on and resolved similar issues. my eta is 4 hours

[grantfox-oss]

🦊 GrantFox — @AugistineCreates has been assigned to this issue as part of the Official Campaign | FWC26 campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #275)
2. Your PR will be reviewed by the ChainForgee maintainers

Good luck! Track your progress on GrantFox.

[grantfox-oss]

🎉 This issue has been marked as completed on GrantFox as part of the Official Campaign | FWC26 campaign!

@AugistineCreates's PR #341 was approved and merged by @kilodesodiq-arch.

🏆 @AugistineCreates: You earned 45 FoxPoints for this contribution! Your current tier: Specialist (3,259 total points). Track your full progress on GrantFox.

👏 Great work, @AugistineCreates! Keep contributing to ChainForgee.