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.

README.md

@@ -72,6 +72,30 @@ Top-level folders become courses on the home page.
 
 5. Open the site, create an account, and watch.
 
+## Quizzes
+
+Course folders may contain quiz text files next to the videos. Files named
+`*_quiz_solved.txt` are indexed (a `*_quiz_notsolved.txt` twin is ignored —
+it's the same content without answers). Format:
+
+```
+[&] Which two are supported CLI connectivity methods? (Select two)
+- RDP
+- rlogin
+- GUI widget -- Correct
+- FortiExplorer -- Correct
+```
+
+Questions start with `[&]`, options with `- `, and correct options end with
+`-- Correct`. Questions with more than one correct option automatically render
+as checkboxes instead of radios.
+
+Quizzes are graded server-side (answers are never sent to the browser before
+submission). A question counts only if the selected set exactly matches the
+correct set. Best score, last score, and attempt count are stored per user;
+a quiz is *completed* once every question is answered correctly in one
+attempt, and quiz scores count toward folder progress alongside videos.
+
 ## How progress works
 
 - The player POSTs `{video_id, position, duration}` to `api/progress.php`