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/components/mentor/file-promotion-panel.tsx

@@ -17,7 +17,7 @@ import { FileText, Upload, CheckCircle2, ArrowUp } from 'lucide-react'
 import { toast } from 'sonner'
 
 interface FilePromotionPanelProps {
-  mentorAssignmentId: string
+  projectId: string
 }
 
 function formatFileSize(bytes: number): string {
@@ -28,14 +28,14 @@ function formatFileSize(bytes: number): string {
   return parseFloat((bytes / Math.pow(k, i)).toFixed(2)) + ' ' + sizes[i]
 }
 
-export function FilePromotionPanel({ mentorAssignmentId }: FilePromotionPanelProps) {
+export function FilePromotionPanel({ projectId }: FilePromotionPanelProps) {
   const [selectedSlot, setSelectedSlot] = useState<string>('')
   const utils = trpc.useUtils()
 
   const { data: workspaceFiles = [], isLoading: filesLoading } =
     trpc.mentor.workspaceGetFiles.useQuery(
-      { mentorAssignmentId },
-      { enabled: !!mentorAssignmentId },
+      { projectId },
+      { enabled: !!projectId },
     )
 
   const promoteMutation = trpc.mentor.workspacePromoteFile.useMutation({