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/app/Markdown/MediaUrlResolver.php

@@ -55,9 +55,49 @@ private function detectYouTube(string $url): ?string
             return null;
         }
         $src = "https://www.youtube-nocookie.com/embed/{$videoId}";
+        $start = $this->extractYouTubeStart($url);
+        if ($start !== null) {
+            $src .= "?start={$start}";
+        }
         return $this->iframeHtml($src, 'youtube');
     }
 
+    private function extractYouTubeStart(string $url): ?int
+    {
+        if (preg_match('/[?&]t=([^&#]+)/', $url, $m)) {
+            $seconds = $this->parseTimestamp($m[1]);
+            if ($seconds !== null) {
+                return $seconds;
+            }
+        }
+        if (preg_match('/[?&]start=(\d+)/', $url, $m)) {
+            return (int) $m[1];
+        }
+        return null;
+    }
+
+    private function parseTimestamp(string $t): ?int
+    {
+        if (ctype_digit($t)) {
+            return (int) $t;
+        }
+        $total = 0;
+        $matched = false;
+        if (preg_match('/(\d+)h/', $t, $m)) {
+            $total += (int) $m[1] * 3600;
+            $matched = true;
+        }
+        if (preg_match('/(\d+)m/', $t, $m)) {
+            $total += (int) $m[1] * 60;
+            $matched = true;
+        }
+        if (preg_match('/(\d+)s/', $t, $m)) {
+            $total += (int) $m[1];
+            $matched = true;
+        }
+        return $matched ? $total : null;
+    }
+
     private function iframeHtml(string $src, string $provider): string
     {
         $safe = htmlspecialchars($src, ENT_QUOTES, 'UTF-8');

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],
+        ];
+    }
 }