feat(mentor-workspace): re-scope files from assignment to project for team-wide visibility

- 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.

tests/unit/mentor-upload-token.test.ts

@@ -6,6 +6,7 @@ import {
 
 const samplePayload: MentorUploadPayload = {
   mentorAssignmentId: 'ma-123',
+  projectId: 'proj-789',
   uploaderUserId: 'user-456',
   fileName: 'doc.pdf',
   mimeType: 'application/pdf',