Fix HTTP 500 on PHP < 8.2: drop standalone true union type

registerUser() declared `: string|true`; the bare `true` type requires
PHP 8.2, so including auth.php was a fatal error on PHP 8.0/8.1 — every
login/API call returned 500 while install.php (which doesn't include
auth.php) worked. Use string|bool instead. Also install a JSON exception
handler so future server errors surface a message to the SPA instead of
a blank 500.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

README.md

@@ -1,6 +1,6 @@
 # CourseHub
 
-Self-hosted course video player: PHP 8 + MySQL/MariaDB + Bootstrap 5 + [Plyr](https://github.com/sampotts/plyr).
+Self-hosted course video player: PHP 8.0+ + MySQL/MariaDB + Bootstrap 5 + [Plyr](https://github.com/sampotts/plyr).
 
 Single-page app: `index.php` is a static shell; all navigation, login, browsing,
 playback, and scanning happen over AJAX against the JSON API in `api/`

includes/auth.php

@@ -51,7 +51,8 @@ function attemptLogin(string $username, string $password): bool
     return false;
 }
 
-function registerUser(string $username, string $password): string|true
+/** Returns true on success, or an error-message string. */
+function registerUser(string $username, string $password): string|bool
 {
     if (!preg_match('/^[A-Za-z0-9_.-]{3,50}$/', $username)) {
         return 'Username must be 3-50 characters (letters, digits, _ . -).';

includes/db.php

@@ -1,6 +1,19 @@
 <?php
 require_once __DIR__ . '/../config.php';
 
+// Uncaught exceptions in web requests become JSON errors (the SPA surfaces
+// them) instead of blank 500 pages; details still go to the PHP error log.
+if (PHP_SAPI !== 'cli') {
+    set_exception_handler(function (Throwable $e): void {
+        error_log($e);
+        http_response_code(500);
+        if (!headers_sent()) {
+            header('Content-Type: application/json');
+        }
+        echo json_encode(['error' => 'Server error: ' . $e->getMessage()]);
+    });
+}
+
 function db(): PDO
 {
     static $pdo = null;