test: add property-based test for AES envelope round-trip

[AugistineCreates]

Closes #275

This PR introduces a property-based test utilizing fast-check to assert that the AES-256-GCM buffer encryption/decryption round-trip (the AES envelope) preserves checksum equality across the full range of evidence sizes from 1 KB up to 100 MB.

Key Changes

• Dependency: Added fast-check (^4.9.0) to app/backend/package.json devDependencies.
• Tests: Implemented property-based test in [upload-roundtrip.spec.ts]( covering sizes between 1 KB and 100 MB.

Implementation Details

• Log-Uniform Distribution Mapping: To meet the CI performance budget of under 30 seconds for 1000 random sequences, the double value from fast-check is mapped using a piecewise log-uniform distribution. This biases testing toward small/medium buffers (90% between 1 KB and 1 MB) while still covering large uploads (10% between 1 MB and 100 MB).
• Fast Buffer Construction: Instead of relying on slow random byte generators for large buffers, a random 4 KB pattern is repeated to fill the buffer, which significantly speeds up initialization.
• Verification: Tests run successfully locally and complete in ~21 seconds for 1000 runs.

[kilodesodiq-arch]

Hi 👋 Appreciate this PR! The CI is flagging a failing check — could you take a peek at the failed job and push a fix? Happy to help if you need a hand 🙏