student_id-FK-Migration Stufe 1+2: Name-Joins -> student_sessions.student_id

- student_sessions: neue Spalte student_id (+Index), backfilled aus (class_id,display_name)
- Session::createStudent setzt student_id (per Lookup, falls nicht uebergeben)
- alle Joins students<->student_sessions von display_name auf student_id umgestellt:
  Benchmark.php, live.php (5x), results.php (4x + runs), modules.php, profile.php, progress.php (answers+runs)
- verlustfrei verifiziert (alt-Join == neu-Join, class 99/102)

Robuster gegen Umbenennung/Doppelnamen, indexierbare Joins.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

App/php/api/modules.php

@@ -74,7 +74,7 @@ if ($method === 'GET') {
 
         // Individuelle Overrides für diesen Schüler
         $studentRow = $db->fetchOne(
-            'SELECT s.id FROM students s JOIN student_sessions ss ON ss.class_id = s.class_id AND ss.display_name = s.display_name WHERE ss.id = ?',
+            'SELECT student_id AS id FROM student_sessions WHERE id = ?',
             [$sessionId]
         );
         $overrides = [];