feat: add YouTube engine with config file and env support

YouTube Data API v3 engine:
- Add YouTubeConfig to EnginesConfig with api_key field
- Add YOUTUBE_API_KEY env override
- Thread *config.Config through search service to factory
- Factory falls back to env vars if config fields are empty
- Update config.example.toml with youtube section

Also update default local_ported to include google and youtube.

cmd/kafka/main.go

@@ -53,9 +53,10 @@ func main() {
 	}
 
 	svc := search.NewService(search.ServiceConfig{
-		UpstreamURL: cfg.Upstream.URL,
-		HTTPTimeout: cfg.HTTPTimeout(),
-		Cache:       searchCache,
+		UpstreamURL:   cfg.Upstream.URL,
+		HTTPTimeout:   cfg.HTTPTimeout(),
+		Cache:         searchCache,
+		EnginesConfig: cfg,
 	})
 
 	acSvc := autocomplete.NewService(cfg.Upstream.URL, cfg.HTTPTimeout())