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/auth.php

@@ -1,14 +1,40 @@
 <?php
 require_once __DIR__ . '/db.php';
 
+/**
+ * Whether the visitor reached us over HTTPS. Behind the proxy that
+ * CLIENT_IP_HEADER already declares trustworthy (Cloudflare), the origin
+ * connection may be plain HTTP while the visitor's is TLS; the proxy says so in
+ * X-Forwarded-Proto. Without that setting only the connection itself counts —
+ * the header is spoofable by anyone who can reach the origin directly.
+ */
+function requestIsHttps(): bool
+{
+    if (!empty($_SERVER['HTTPS']) && $_SERVER['HTTPS'] !== 'off') {
+        return true;
+    }
+    if (defined('CLIENT_IP_HEADER') && CLIENT_IP_HEADER !== '') {
+        $proto = strtolower(trim(explode(',', $_SERVER['HTTP_X_FORWARDED_PROTO'] ?? '')[0]));
+        return $proto === 'https';
+    }
+    return false;
+}
+
 if (session_status() === PHP_SESSION_NONE) {
+    // SESSION_LIFETIME (seconds) keeps people signed in across browser restarts
+    // and idle stretches. 0 / undefined = a browser-session cookie plus the
+    // php.ini idle limit (24 min by default — short for a video app).
+    $lifetime = defined('SESSION_LIFETIME') ? max(0, (int)SESSION_LIFETIME) : 0;
+    if ($lifetime > 0) {
+        ini_set('session.gc_maxlifetime', (string)$lifetime);
+    }
     // Reject session IDs the server never issued (blocks session fixation),
     // and keep the cookie away from scripts and cross-site requests.
     ini_set('session.use_strict_mode', '1');
     session_set_cookie_params([
-        'lifetime' => 0,
+        'lifetime' => $lifetime,
         'path'     => '/',
-        'secure'   => !empty($_SERVER['HTTPS']) && $_SERVER['HTTPS'] !== 'off',
+        'secure'   => requestIsHttps(),
         'httponly' => true,
         'samesite' => 'Lax',
     ]);
@@ -52,16 +78,6 @@ function currentUsername(): ?string
     return $_SESSION['username'] ?? null;
 }
 
-function requireLogin(): int
-{
-    $uid = currentUserId();
-    if ($uid === null) {
-        header('Location: login.php');
-        exit;
-    }
-    return $uid;
-}
-
 function requireLoginApi(): int
 {
     $uid = currentUserId();
@@ -198,7 +214,14 @@ function registerUser(string $username, string $password): string|bool
     if ($stmt->fetch()) {
         return 'That username is already taken.';
     }
-    $stmt = db()->prepare('INSERT INTO users (username, password_hash) VALUES (?, ?)');
-    $stmt->execute([$username, password_hash($password, PASSWORD_DEFAULT)]);
+    try {
+        $stmt = db()->prepare('INSERT INTO users (username, password_hash) VALUES (?, ?)');
+        $stmt->execute([$username, password_hash($password, PASSWORD_DEFAULT)]);
+    } catch (PDOException $e) {
+        if ($e->getCode() === '23000') {      // lost the race to the same name
+            return 'That username is already taken.';
+        }
+        throw $e;
+    }
     return true;
 }