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/results.php

@@ -206,7 +206,7 @@ if ($view === 'matrix') {
         "SELECT s.id AS student_id, a.sim_id, a.results, a.process_log
            FROM assessments a
            JOIN student_sessions ss ON ss.id = a.session_id
-           JOIN students s ON s.class_id = ss.class_id AND s.display_name = ss.display_name
+           JOIN students s ON s.id = ss.student_id
           WHERE a.class_id = ? AND s.deleted_at IS NULL
           ORDER BY s.id, a.sim_id, a.submitted_at DESC",
         [$classId]
@@ -387,7 +387,7 @@ if ($view === 'fleiss') {
                 COUNT(*) AS sessions
            FROM assessments a
            JOIN student_sessions ss ON ss.id = a.session_id
-           JOIN students s ON s.class_id = ss.class_id AND s.display_name = ss.display_name
+           JOIN students s ON s.id = ss.student_id
           WHERE a.class_id = ? AND $sinceSql AND s.deleted_at IS NULL $modSql
           GROUP BY s.id, bucket, a.sim_id",
         $params
@@ -553,7 +553,7 @@ if ($view === 'activity') {
             "SELECT $bucketA AS bucket, s.id AS student_id, s.display_name AS name, s.emoji_avatar AS emoji, COUNT(*) AS n
                FROM assessments a
                JOIN student_sessions ss ON ss.id = a.session_id
-               JOIN students s ON s.class_id = ss.class_id AND s.display_name = ss.display_name
+               JOIN students s ON s.id = ss.student_id
               WHERE a.class_id = ? AND $sinceSqlA AND s.deleted_at IS NULL
               GROUP BY bucket, s.id",
             [$classId]
@@ -656,10 +656,10 @@ if ($view === 'module') {
             "SELECT a.id, a.submitted_at, a.duration_ms, a.results, a.reflections
                FROM assessments a
                JOIN student_sessions ss ON ss.id = a.session_id
-              WHERE ss.class_id = ? AND ss.display_name = ? AND a.sim_id = ?
+              WHERE ss.student_id = ? AND a.sim_id = ?
               ORDER BY a.submitted_at DESC
               LIMIT 100",
-            [$classId, $student['display_name'], $moduleId]
+            [$studentId, $moduleId]
         );
         // results + reflections JSON parsen (Frontend sieht's lieber dekodiert)
         foreach ($runs as &$run) {
@@ -851,7 +851,7 @@ if ($view === 'reflections') {
                    ss.display_name, s.id AS student_id, s.emoji_avatar
               FROM assessments a
               JOIN student_sessions ss ON ss.id = a.session_id
-         LEFT JOIN students s ON s.class_id = ss.class_id AND s.display_name = ss.display_name AND s.deleted_at IS NULL
+         LEFT JOIN students s ON s.id = ss.student_id AND s.deleted_at IS NULL
              WHERE a.class_id = ?
                AND a.reflections IS NOT NULL
                AND JSON_LENGTH(a.reflections) > 0";