Scaffold MediaUrlResolver with null fallback

Initial skeleton returning null for any non-media URL. Subsequent commits
add detection for video, audio, YouTube, and Vimeo.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/app/Markdown/MediaUrlResolver.php

@@ -0,0 +1,14 @@
+<?php
+
+namespace App\Markdown;
+
+class MediaUrlResolver
+{
+    public function resolve(string $url): ?string
+    {
+        if ($url === '') {
+            return null;
+        }
+        return null;
+    }
+}

src/tests/Unit/Markdown/MediaUrlResolverTest.php

@@ -0,0 +1,38 @@
+<?php
+
+namespace Tests\Unit\Markdown;
+
+use App\Markdown\MediaUrlResolver;
+use PHPUnit\Framework\Attributes\DataProvider;
+use Tests\TestCase;
+
+class MediaUrlResolverTest extends TestCase
+{
+    private MediaUrlResolver $resolver;
+
+    protected function setUp(): void
+    {
+        parent::setUp();
+        $this->resolver = new MediaUrlResolver();
+    }
+
+    #[DataProvider('nonMediaUrls')]
+    public function test_returns_null_for_non_media_urls(string $url): void
+    {
+        $this->assertNull($this->resolver->resolve($url));
+    }
+
+    public static function nonMediaUrls(): array
+    {
+        return [
+            'normal image' => ['/photo.jpg'],
+            'svg' => ['/icon.svg'],
+            'png' => ['/avatar.png'],
+            'no extension' => ['/foo'],
+            'empty string' => [''],
+            'javascript scheme' => ['javascript:alert(1)'],
+            'host-only' => ['http://'],
+            'youtu.be lookalike host' => ['https://example.com/youtu.be-fake/abc'],
+        ];
+    }
+}