feat: add YouTube engine via Data API v3

Uses the official YouTube Data API v3. Requires YOUTUBE_API_KEY
environment variable (free from Google Cloud Console).

Returns video results with title, description, channel, publish
date, and thumbnail URL. Falls back gracefully if no API key.

internal/engines/factory.go

@@ -32,6 +32,10 @@ func NewDefaultPortedEngines(client *http.Client) map[string]Engine {
 		"github":     &GitHubEngine{client: client},
 		"reddit":     &RedditEngine{client: client},
 		"bing":   &BingEngine{client: client},
-		"google": &GoogleEngine{client: client},
+		"google":  &GoogleEngine{client: client},
+		"youtube": &YouTubeEngine{
+			client:  client,
+			baseURL: "https://www.googleapis.com",
+		},
 	}
 }