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.

internal/engines/factory.go

@@ -4,23 +4,42 @@ import (
 	"net/http"
 	"os"
 	"time"
+
+	"github.com/metamorphosis-dev/kafka/internal/config"
 )
 
 // NewDefaultPortedEngines returns the starter set of Go-native engines.
 // The service can swap/extend this registry later as more engines are ported.
-func NewDefaultPortedEngines(client *http.Client) map[string]Engine {
+// If cfg is nil, falls back to reading API keys from environment variables.
+func NewDefaultPortedEngines(client *http.Client, cfg *config.Config) map[string]Engine {
 	if client == nil {
 		client = &http.Client{Timeout: 10 * time.Second}
 	}
 
+	var braveAPIKey, braveAccessToken, youtubeAPIKey string
+	if cfg != nil {
+		braveAPIKey = cfg.Engines.Brave.APIKey
+		braveAccessToken = cfg.Engines.Brave.AccessToken
+		youtubeAPIKey = cfg.Engines.YouTube.APIKey
+	}
+	if braveAPIKey == "" {
+		braveAPIKey = os.Getenv("BRAVE_API_KEY")
+	}
+	if braveAccessToken == "" {
+		braveAccessToken = os.Getenv("BRAVE_ACCESS_TOKEN")
+	}
+	if youtubeAPIKey == "" {
+		youtubeAPIKey = os.Getenv("YOUTUBE_API_KEY")
+	}
+
 	return map[string]Engine{
 		"wikipedia": &WikipediaEngine{client: client},
 		"arxiv":     &ArxivEngine{client: client},
 		"crossref":  &CrossrefEngine{client: client},
 		"braveapi": &BraveEngine{
 			client:          client,
-			apiKey:          os.Getenv("BRAVE_API_KEY"),
-			accessGateToken: os.Getenv("BRAVE_ACCESS_TOKEN"),
+			apiKey:          braveAPIKey,
+			accessGateToken: braveAccessToken,
 			resultsPerPage: 20,
 		},
 		"qwant": &QwantEngine{
@@ -35,6 +54,7 @@ func NewDefaultPortedEngines(client *http.Client) map[string]Engine {
 		"google":  &GoogleEngine{client: client},
 		"youtube": &YouTubeEngine{
 			client:  client,
+			apiKey:  youtubeAPIKey,
 			baseURL: "https://www.googleapis.com",
 		},
 	}