feat: add Google engine (experimental, may be blocked)

Google blocks scrapers — results depend on whether Google serves
a static page or a JS challenge. Set LOCAL_PORTED_ENGINES=google
to enable. Without it, Google is proxied to upstream SearXNG.

Closes #1

internal/engines/factory.go

@@ -31,6 +31,7 @@ func NewDefaultPortedEngines(client *http.Client) map[string]Engine {
 		"duckduckgo": &DuckDuckGoEngine{client: client},
 		"github":     &GitHubEngine{client: client},
 		"reddit":     &RedditEngine{client: client},
-		"bing":       &BingEngine{client: client},
+		"bing":   &BingEngine{client: client},
+		"google": &GoogleEngine{client: client},
 	}
 }