Show full Grid-Review in Ground Truth step + GT badge in session list

- StepGroundTruth now shows the split view (original image + table)
  so the user can verify the final result before marking as GT
- Backend session list now returns is_ground_truth flag
- SessionList shows amber "GT" badge for marked sessions

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

admin-lehrer/app/(admin)/ai/ocr-pipeline/types.ts

@@ -36,6 +36,7 @@ export interface SessionListItem {
   parent_session_id?: string
   document_group_id?: string
   page_number?: number
+  is_ground_truth?: boolean
   created_at: string
   updated_at?: string
 }