Add video.php helper referenced in composer autoload

Fixes deploy build failure: composer dump-autoload couldn't find
app/Helpers/video.php which is required by composer.json autoload.files.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/Helpers/video.php

+<?php
+
+if (!function_exists('video_embed_url')) {
+    function video_embed_url(string $url): ?string
+    {
+        if (preg_match('/(?:youtube\.com\/watch\?v=|youtu\.be\/)([\w-]+)/', $url, $m)) {
+            return "https://www.youtube.com/embed/{$m[1]}";
+        }
+
+        if (preg_match('/vimeo\.com\/(\d+)/', $url, $m)) {
+            return "https://player.vimeo.com/video/{$m[1]}";
+        }
+
+        return null;
+    }
+}