- MentorFile.projectId is the new access boundary; mentorAssignmentId stays
as informational audit FK (nullable).
- uploadFile derives projectId from the assignment; getFiles takes projectId
directly; deleteFile/addFileComment auth checks any mentor on the project
OR a project team member.
- HMAC upload token now binds to projectId (in addition to assignmentId).
- promoteFile reads file.projectId directly (no more mentorAssignment null
navigation).
- Removes 3 placeholder NOT_FOUND guards added in Task 4.