feat(exam): regrade action + stop inviting a re-take on a finished exam

Two fixes surfaced by actual candidate-flow testing:

- Portal 'My registrations' always showed 'Take exam' regardless of whether
  the candidate had already finished — clicking it just hit
  attempt_already_submitted server-side. The registrations/mine response
  now carries the attempt's status; the row shows a Completed/Time expired
  badge once finished, or 'Resume exam' while still in progress, and only
  invites a fresh start when there's genuinely nothing yet.
- Backoffice candidates panel: a Regrade button next to a SUBMITTED/EXPIRED
  sitting, calling the new POST /exam-attempts/:id/regrade — the officer's
  path to the same staff-triggered regrade, no SQL needed.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

apps/portal/src/app/features/exams/pages/ExamsPage/columns.tsx

@@ -94,6 +94,23 @@ export function registrationColumns(deps: {
       header: 'Exam',
       cell: ({ row }) => {
         const exam = row.original.exam;
+        const attemptStatus = row.original.attempt?.status;
+        // Already finished — no restart, no more room for "Take exam" to
+        // invite a click that the backend would just refuse.
+        if (attemptStatus === 'SUBMITTED') {
+          return (
+            <Badge size="sm" variant="light" color="teal">
+              Completed
+            </Badge>
+          );
+        }
+        if (attemptStatus === 'EXPIRED') {
+          return (
+            <Badge size="sm" variant="light" color="red">
+              Time expired
+            </Badge>
+          );
+        }
         const eligible =
           exam?.status === 'ACTIVE' && !NOT_SITTING.includes(row.original.attendanceStatus);
         if (!eligible || !deps.can([PORTAL_PERMISSIONS.APPLY_EXAM])) return null;
@@ -104,7 +121,7 @@ export function registrationColumns(deps: {
             leftSection={<IconPlayerPlay size={13} />}
             onClick={() => deps.onStartExam(row.original)}
           >
-            Take exam
+            {attemptStatus === 'IN_PROGRESS' ? 'Resume exam' : 'Take exam'}
           </Button>
         );
       },

apps/portal/src/app/features/exams/pages/ExamsPage/index.tsx

@@ -54,6 +54,8 @@ export interface MyRegistration {
   attemptNumber: number;
   attendanceStatus: AttendanceStatus;
   exam?: OpenExam;
+  /** The candidate's online sitting, when one has been started. */
+  attempt?: { status: 'IN_PROGRESS' | 'SUBMITTED' | 'EXPIRED' } | null;
 }
 
 export interface MyResult {