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/lib/Session.php

@@ -20,8 +20,10 @@ class Session {
         return $_COOKIE[self::COOKIE_NAME] ?? null;
     }
 
-    /** Neue Schueler-Session erstellen */
-    public static function createStudent(int $classId, string $displayName = ''): string {
+    /** Neue Schueler-Session erstellen. student_id wird direkt gesetzt (per FK
+     *  statt fragilem display_name-Join) — falls nicht übergeben, aus
+     *  (class_id, display_name) aufgelöst. */
+    public static function createStudent(int $classId, string $displayName = '', ?int $studentId = null): string {
         $uuid = sprintf('%s-%s-%s-%s-%s',
             bin2hex(random_bytes(4)),
             bin2hex(random_bytes(2)),
@@ -31,9 +33,16 @@ class Session {
         );
 
         $db = Database::get();
+        if ($studentId === null && $displayName !== '') {
+            $row = $db->fetchOne(
+                'SELECT id FROM students WHERE class_id = ? AND display_name = ? AND deleted_at IS NULL',
+                [$classId, $displayName]
+            );
+            if ($row) $studentId = (int)$row['id'];
+        }
         $db->execute(
-            'INSERT INTO student_sessions (id, class_id, display_name) VALUES (?, ?, ?)',
-            [$uuid, $classId, $displayName]
+            'INSERT INTO student_sessions (id, class_id, display_name, student_id) VALUES (?, ?, ?, ?)',
+            [$uuid, $classId, $displayName, $studentId]
         );
 
         setcookie(self::COOKIE_NAME, $uuid, [