Normalize YouTube timestamp parameters to ?start=N

Accepts ?t=30s, ?t=30, ?t=1m20s, ?t=1h2m3s, and ?start=N. Converts to
seconds and emits as ?start=N on the embed URL. ?t= takes priority over
?start= when both are present.

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

src/tests/Unit/Markdown/MediaUrlResolverTest.php

@@ -126,4 +126,23 @@ public static function invalidYoutubeUrls(): array
             'XSS attempt in id' => ['https://youtu.be/abc"><script>'],
         ];
     }
+
+    #[DataProvider('youtubeTimestampUrls')]
+    public function test_youtube_timestamp_normalizes_to_start(string $url, int $expectedStart): void
+    {
+        $html = $this->resolver->resolve($url);
+        $this->assertNotNull($html);
+        $this->assertStringContainsString("?start={$expectedStart}", $html);
+    }
+
+    public static function youtubeTimestampUrls(): array
+    {
+        return [
+            't=30s' => ['https://youtu.be/dQw4w9WgXcQ?t=30s', 30],
+            't=30 (no suffix)' => ['https://www.youtube.com/watch?v=dQw4w9WgXcQ&t=30', 30],
+            't=1m20s' => ['https://youtu.be/dQw4w9WgXcQ?t=1m20s', 80],
+            't=1h2m3s' => ['https://youtu.be/dQw4w9WgXcQ?t=1h2m3s', 3723],
+            'start=45' => ['https://www.youtube.com/watch?v=dQw4w9WgXcQ&start=45', 45],
+        ];
+    }
 }