Add quiz support: parse *_quiz_solved.txt files, take and grade quizzes

Scanner indexes quiz files found next to videos ([&] questions, "- "
options, "-- Correct" markers; _notsolved twins ignored). Quizzes appear
in the folder content list interleaved with videos by filename order.
New api/quiz.php serves questions without answers and grades attempts
server-side; per-user best/last score and attempts stored in
quiz_progress. Folder/course progress now aggregates videos + quizzes
with partial credit for partial scores.

api/library.php

@@ -24,6 +24,7 @@ function videoSummary(array $v): array
     return [
         'id'        => (int)$v['id'],
         'name'      => $v['name'],
+        'filename'  => $v['filename'],
         'duration'  => $duration !== null ? (float)$duration : null,
         'position'  => $position,
         'completed' => (bool)($v['completed'] ?? false),
@@ -31,6 +32,19 @@ function videoSummary(array $v): array
     ];
 }
 
+function quizSummary(array $q): array
+{
+    return [
+        'id'         => (int)$q['id'],
+        'name'       => $q['name'],
+        'filename'   => $q['filename'],
+        'questions'  => (int)$q['num_questions'],
+        'attempts'   => (int)($q['attempts'] ?? 0),
+        'best_score' => (int)($q['best_score'] ?? 0),
+        'completed'  => (bool)($q['completed'] ?? false),
+    ];
+}
+
 function crumbList(array $folder): array
 {
     return array_map(
@@ -70,6 +84,8 @@ switch ($view) {
                                        childFolders((int)$folder['id'])),
             'videos'      => array_map('videoSummary',
                                        folderVideos((int)$folder['id'], $userId)),
+            'quizzes'     => array_map('quizSummary',
+                                       folderQuizzes((int)$folder['id'], $userId)),
         ]);
         break;