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/(mentor)/mentor/workspace/[projectId]/page.tsx

@@ -104,7 +104,10 @@ export default function MentorWorkspaceDetailPage() {
 
         <TabsContent value="files" className="mt-6">
           {assignment ? (
-            <WorkspaceFilesPanel mentorAssignmentId={assignment.id} />
+            <WorkspaceFilesPanel
+              projectId={projectId}
+              mentorAssignmentId={assignment.id}
+            />
           ) : (
             <Card>
               <CardContent className="text-center py-8">
@@ -117,7 +120,7 @@ export default function MentorWorkspaceDetailPage() {
 
         <TabsContent value="promotion" className="mt-6">
           {assignment ? (
-            <FilePromotionPanel mentorAssignmentId={assignment.id} />
+            <FilePromotionPanel projectId={projectId} />
           ) : (
             <Card>
               <CardContent className="text-center py-8">