Show quiz exhibits, keep sign-ins alive, and close a batch of edge cases

- A quiz question can now carry the diagram it refers to: a Markdown
  image line anywhere inside the question block is parsed out of the
  text and rendered under it (referrerpolicy="no-referrer", lazy). The
  URL points at the course provider, so an offline install gets a note
  and a link instead of an empty box.
- New optional SESSION_LIFETIME (30 days in the example config) replaces
  the browser-session cookie plus php.ini's 24-minute idle limit, which
  signed people out over a long pause. Configs that predate it behave as
  before. Behind a proxy trusted enough for CLIENT_IP_HEADER, the cookie
  is also marked Secure from X-Forwarded-Proto — TLS ending at the proxy
  no longer costs the flag.
- Error responses no longer echo the exception message unless DEBUG: a
  PDOException names the DB user, host and tables, and that handler also
  runs for visitors who never signed in.
- The .htaccess denials are anchored to a path segment rather than the
  root, so videos/ and includes/ stay unreachable under a prefix such as
  /coursehub/ instead of silently opening up.
- A rescan takes a MySQL advisory lock; a second one answers "already
  running" rather than racing the first and flagging its rows missing. A
  file swapped in at the same path with a different size drops its
  cached duration so the probe reads the new length.
- stream.php discards output buffering and zlib compression (both break
  the promised Content-Length) and answers HEAD with headers alone.
- A progress save with no duration keeps the length already stored
  instead of nulling it, and the folder view builds its rollup once.
- In the SPA, a view whose data arrives after the route moved on is
  discarded — previously a slow load could paint over a newer screen or
  start a heartbeat for a video nobody was watching. Also: the quiz
  submit button can't be double-fired, leaving a video during the
  autoplay countdown no longer jumps ahead later, the final position is
  always saved on 'ended', the playlist scrolls the current entry into
  its own box, the login field takes focus, views start at the top, and
  a stray "%" in the hash no longer throws.
- Dropped the unused requireLogin(); registerUser() answers "taken"
  instead of a 500 when two people race for the same name.

Claude-Session: https://claude.ai/code/session_01AXMrtWNUV7NU1pCUHBG4t1

includes/functions.php

@@ -83,9 +83,13 @@ function mp4Duration(string $path): ?float
  * Quiz file parser
  * Format ("*_quiz_solved.txt"):
  *   [&] Question text (Select two)
+ *   ![exhibit](https://example.com/diagram.png)      <- optional, one per question
  *   - wrong option
  *   - right option -- Correct
- * Returns [['text', 'options' => [...], 'correct' => [optIndex,...], 'multi'], ...]
+ * A Markdown image line anywhere inside a question is its exhibit (the
+ * diagram the question refers to); it is kept out of the question text and
+ * returned as ['url', 'alt'] so the UI can show the picture.
+ * Returns [['text', 'image' => ?[...], 'options' => [...], 'correct' => [optIndex,...], 'multi'], ...]
  * ------------------------------------------------------------------------- */
 function parseQuizFile(string $absPath): array
 {
@@ -100,7 +104,9 @@ function parseQuizFile(string $absPath): array
 
         if (str_starts_with($line, '[&]')) {
             if ($cur && $cur['options']) $questions[] = $cur;
-            $cur = ['text' => trim(substr($line, 3)), 'options' => [], 'correct' => []];
+            $cur = ['text' => trim(substr($line, 3)), 'image' => null, 'options' => [], 'correct' => []];
+        } elseif ($cur !== null && preg_match('/^!\[([^\]]*)\]\((https?:\/\/[^\s)]+)\)$/i', $line, $m)) {
+            $cur['image'] = ['url' => $m[2], 'alt' => trim($m[1]) !== '' ? trim($m[1]) : 'Exhibit'];
         } elseif (str_starts_with($line, '-') && $cur !== null) {
             $opt = trim(substr($line, 1));
             if (preg_match('/\s*--\s*correct\s*$/i', $opt)) {
@@ -135,6 +141,23 @@ function scanLibrary(): array
     }
 
     $db = db();
+
+    // One scan at a time. Two overlapping scans would upsert the same rows
+    // inside two transactions and then each try to flag the other's work
+    // missing; the lock is per-connection and released below whatever happens.
+    if ((int)$db->query("SELECT GET_LOCK('coursehub_scan', 0)")->fetchColumn() !== 1) {
+        throw new RuntimeException('A scan is already running — try again in a moment.');
+    }
+    try {
+        return scanLibraryLocked($db, $root);
+    } finally {
+        $db->query("SELECT RELEASE_LOCK('coursehub_scan')");
+    }
+}
+
+/** The scan proper — only ever called by scanLibrary() while it holds the lock. */
+function scanLibraryLocked(PDO $db, string $root): array
+{
     $stats = ['folders' => 0, 'videos' => 0, 'quizzes' => 0,
               'missing_folders' => 0, 'missing_videos' => 0, 'missing_quizzes' => 0];
     $seenFolderIds = [];
@@ -148,10 +171,17 @@ function scanLibrary(): array
              depth = VALUES(depth), sort_order = VALUES(sort_order), is_missing = 0,
              id = LAST_INSERT_ID(id)'
     );
+    // A file replaced by a different one at the same path (re-encoded, a
+    // corrected upload) is a different length: forget the cached duration so
+    // the probe below reads it afresh. The assignment order matters — MySQL
+    // evaluates ON DUPLICATE KEY UPDATE left to right, so the comparison has
+    // to happen before size_bytes is overwritten.
     $upsertVideo = $db->prepare(
         'INSERT INTO videos (folder_id, name, filename, path, size_bytes, sort_order, is_missing)
          VALUES (?, ?, ?, ?, ?, ?, 0)
-         ON DUPLICATE KEY UPDATE folder_id = VALUES(folder_id), name = VALUES(name),
+         ON DUPLICATE KEY UPDATE
+             duration_seconds = IF(size_bytes <> VALUES(size_bytes), NULL, duration_seconds),
+             folder_id = VALUES(folder_id), name = VALUES(name),
              filename = VALUES(filename), size_bytes = VALUES(size_bytes),
              sort_order = VALUES(sort_order), is_missing = 0,
              id = LAST_INSERT_ID(id)'