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/backoffice/src/app/features/exam/api/exam-api.ts

@@ -11,6 +11,7 @@ import type {
   ExamIncident,
   CreateIncidentPayload,
   ResolveIncidentPayload,
+  RegradeOutcome,
 } from '../types/exam';
 
 const examApi = baseApi.injectEndpoints({
@@ -92,6 +93,14 @@ const examApi = baseApi.injectEndpoints({
       }),
       invalidatesTags: ['Api'],
     }),
+    /** Staff-triggered re-run of auto-grading for one finalized attempt. */
+    regradeAttempt: builder.mutation<RegradeOutcome, string>({
+      query: (attemptId) => ({
+        url: `/exam-attempts/${attemptId}/regrade`,
+        method: 'POST',
+      }),
+      invalidatesTags: ['Api'],
+    }),
   }),
   overrideExisting: false,
 });
@@ -109,4 +118,5 @@ export const {
   useGetExamIncidentsQuery,
   useRecordIncidentMutation,
   useResolveIncidentMutation,
+  useRegradeAttemptMutation,
 } = examApi;