Fix blank page from stale cached assets; surface errors

A browser or CDN holding an assets/app.js from before the shell's element
IDs were renamed runs it against the newer index.php, where it grabs
btn-logout/btn-rescan/nav-user/nav-username/app — all now null — and throws
before it can unhide #app-shell. The result is a blank page whose only clue
is a TypeError in the console.

- index.php: asset() stamps local css/js with the file mtime, so a deploy is
  a new URL and a stale copy can't be paired with a newer shell.
- index.php: install an error handler ahead of every other script (capture
  phase, so failed resource loads are caught too) that paints uncaught
  errors, rejections and load failures into a #boot-error panel. It works
  even when app.js itself is what failed, which is this case.
- includes/db.php: add a register_shutdown_function fatal handler, since
  set_exception_handler never sees parse errors, exhausted memory or a
  failed require — those were ending requests with an empty body. Both
  handlers flush partial output and answer in JSON; display_errors stays off
  so a warning can't corrupt a JSON body.
- New optional DEBUG config flag adds exception type, file:line and trace to
  those responses. config.php is gitignored, so installs without the setting
  default to false.

Claude-Session: https://claude.ai/code/session_014oyi9DMdhALrGz989MNNZJ

index.php

@@ -1,4 +1,21 @@
-<?php /* CourseHub SPA shell — all data flows through api/ via fetch. */ ?>
+<?php
+/* CourseHub SPA shell — all data flows through api/ via fetch. */
+
+/**
+ * Version a local asset by its mtime.
+ *
+ * Browsers and CDNs (Cloudflare in front of this app) cache assets/*.css|js
+ * indefinitely otherwise, and a stale app.js against a newer shell fails hard:
+ * it grabs element IDs that no longer exist, throws before it can unhide the
+ * shell, and leaves a blank page with nothing in the markup to explain it.
+ * Stamping the mtime means a deploy is a new URL, so that can't happen.
+ */
+function asset(string $path): string
+{
+    $mtime = @filemtime(__DIR__ . '/' . $path);
+    return $path . '?v=' . ($mtime ?: '0');
+}
+?>
 <!doctype html>
 <html lang="en">
 <head>
@@ -10,7 +27,38 @@
 <link rel="preconnect" href="https://fonts.gstatic.com" crossorigin>
 <link href="https://fonts.googleapis.com/css2?family=Inter:wght@400;500;600;700&display=swap" rel="stylesheet">
 <link href="https://cdn.jsdelivr.net/npm/plyr@3.7.8/dist/plyr.css" rel="stylesheet">
-<link href="assets/style.css" rel="stylesheet">
+<link href="<?= asset('assets/style.css') ?>" rel="stylesheet">
+
+<!-- Installed before every other script so it can report failures in them
+     (including assets/app.js itself). Without it a JS error leaves the shell
+     hidden and the page simply blank, with the reason only in the console. -->
+<script>
+(function () {
+  var box = null;
+  function report(kind, detail) {
+    if (!box) {
+      box = document.createElement('pre');
+      box.id = 'boot-error';
+      box.textContent = 'CourseHub failed to start.\n\n';
+      (document.body || document.documentElement).appendChild(box);
+    }
+    box.textContent += kind + ': ' + detail + '\n';
+  }
+  // Capture phase, because resource load failures don't bubble.
+  window.addEventListener('error', function (e) {
+    var el = e.target;
+    if (el && el !== window && (el.src || el.href)) {
+      report('Failed to load', el.src || el.href);
+    } else {
+      report('Error', (e.message || e.error) +
+             ' (' + (e.filename || '?') + ':' + (e.lineno || '?') + ')');
+    }
+  }, true);
+  window.addEventListener('unhandledrejection', function (e) {
+    report('Unhandled rejection', (e.reason && e.reason.message) || e.reason);
+  });
+})();
+</script>
 </head>
 <body>
 
@@ -80,6 +128,6 @@
 
 <script src="https://cdn.jsdelivr.net/npm/plyr@3.7.8/dist/plyr.min.js"></script>
 <script src="https://cdn.jsdelivr.net/npm/animejs@3.2.2/lib/anime.min.js"></script>
-<script src="assets/app.js"></script>
+<script src="<?= asset('assets/app.js') ?>"></script>
 </body>
 </html>