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.

src/app/(applicant)/applicant/mentor/page.tsx

@@ -139,8 +139,9 @@ export default function ApplicantMentorPage() {
       )}
 
       {/* Files */}
-      {primaryAssignment?.id && (
+      {primaryAssignment?.id && projectId && (
         <WorkspaceFilesPanel
+          projectId={projectId}
           mentorAssignmentId={primaryAssignment.id}
           asApplicant
         />