fix: bound attachment filenames

[vycdev2]

Summary

• cap downloaded attachment filenames at a filesystem-safe UTF-8 byte length
• preserve useful name prefixes and extensions while adding a deterministic hash to avoid truncation collisions
• reject directory paths before bounding the validated basename, preserving current download-size and symlink protections

Verification

• npm test — passed: TypeScript build and all 111 tests
• git diff --check main...HEAD — passed

Risk

• Low: the change is isolated to local attachment path generation; short filenames remain unchanged, and regression coverage includes the reported normalization/truncation path.

This was generated by an AI agent (vycdev2). Please verify any changes before merging or applying.

[vycdev]

Found one path-safety blocker. The normal filename and traversal tests pass, but truncation can alter path normalization after the existing validation.

[vycdev]

The resolved path is validated before boundFilename() changes it, so the final path can escape IMAGES_DIR. I reproduced this with "../" + "a".repeat(300) + "/../images/safe.png": the original path normalizes inside images, passes the check, then truncation removes the suffix that brought it back inside and fs.writeFileSync writes into the parent directory. Please validate the final bounded path (or bound only a validated basename) and add this normalization/truncation case as a regression test.

[vycdev2]

Addressed the path-safety review by rebuilding the change on current main, rejecting directory paths before bounding the validated basename, preserving the existing download-size and symlink protections, and adding the reported normalization/truncation case as a regression test. npm test passes all 111 tests.

This was generated by an AI agent (vycdev2). Please verify any changes before merging or applying.