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/i18n/locales/am.ts

@@ -325,6 +325,10 @@ export const am: Translations = {
       retake: "ድጋሚ {{n}}",
       firstSitting: "የመጀመሪያ ሙከራ",
       remarkRequired: "ለመውጣት ወይም ለመታገድ ምክንያት ያስፈልጋል።",
+      regrade: "እንደገና ደረጃ ስጥ",
+      regraded: "ውጤት ከተመዘገበው ሙከራ ተፈጥሯል።",
+      regradeNotEligible: "በራስ-ሰር ሊገመገም አይችልም፦ {{reason}}። ውጤት መዝግብ ተጠቀም።",
+      regradeError: "ይህን ሙከራ እንደገና መገምገም አልተቻለም።",
     },
     attendance: {
       REGISTERED: "አልተጠራም",

apps/backoffice/src/app/i18n/locales/en.ts

@@ -322,6 +322,10 @@ export const en = {
       retake: 'Retake {{n}}',
       firstSitting: 'First sitting',
       remarkRequired: 'A reason is required for a withdrawal or a disqualification.',
+      regrade: 'Regrade',
+      regraded: 'Result created from the graded attempt.',
+      regradeNotEligible: 'Not auto-gradable: {{reason}}. Use Record Result instead.',
+      regradeError: 'Could not regrade this attempt.',
     },
     attendance: {
       REGISTERED: 'Not called',