feat(exam): S4-11 marking flow, results table, CSV, ResultsWidget

- /exam-marker/:batchId/mark — ExamMarkingPage: student queue, per-part mark entry, upsert marks
- /exam-marker/:batchId/results — ExamResultsPage: results table with absent rows, CSV download
- ResultsWidget on ClassDetailPage: last batch summary, class average, absent count, batch creation
- New types: MarkingBatch, StudentSubmission, BatchQueueResponse, BatchResultsResponse, MarkUpsertPayload
- New repo methods: createBatch, listBatches, getBatchQueue, getBatchResults, getBatchCsv, upsertMark

Co-Authored-By: Claude Sonnet 4.6 <[email protected]>

src/pages/timetable/ClassDetailPage.tsx

@@ -9,6 +9,7 @@ import {
     ArrowBack, PersonAdd, PersonRemove, CheckCircle, Cancel, School,
 } from '@mui/icons-material';
 import { useAuth } from '../../contexts/AuthContext';
+import { ResultsWidget } from '../exam';
 
 const API_BASE = import.meta.env.VITE_API_BASE || import.meta.env.VITE_API_URL || '/api';
 
@@ -254,6 +255,8 @@ const ClassDetailPage: React.FC = () => {
                 </Alert>
             )}
 
+            <ResultsWidget classId={cls.id} className={cls.name} />
+
             {/* Tabs */}
             <Tabs value={tab} onChange={(_, v) => setTab(v)} sx={{ borderBottom: 1, borderColor: 'divider', mb: 2 }}>
                 <Tab label={`Students (${cls.student_count})`} />