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/config/config.go

@@ -35,6 +35,7 @@ type EnginesConfig struct {
 	LocalPorted []string    `toml:"local_ported"`
 	Brave       BraveConfig `toml:"brave"`
 	Qwant       QwantConfig `toml:"qwant"`
+	YouTube     YouTubeConfig `toml:"youtube"`
 }
 
 // CacheConfig holds Valkey/Redis cache settings.
@@ -85,6 +86,10 @@ type QwantConfig struct {
 	ResultsPerPage int    `toml:"results_per_page"`
 }
 
+type YouTubeConfig struct {
+	APIKey string `toml:"api_key"`
+}
+
 // Load reads configuration from the given TOML file path.
 // If the file does not exist, it returns defaults (empty values where applicable).
 // Environment variables are used as fallbacks for any zero-value fields.
@@ -109,7 +114,7 @@ func defaultConfig() *Config {
 		},
 		Upstream: UpstreamConfig{},
 		Engines: EnginesConfig{
-			LocalPorted: []string{"wikipedia", "arxiv", "crossref", "braveapi", "qwant", "duckduckgo", "github", "reddit", "bing"},
+			LocalPorted: []string{"wikipedia", "arxiv", "crossref", "braveapi", "qwant", "duckduckgo", "github", "reddit", "bing", "google", "youtube"},
 			Qwant: QwantConfig{
 				Category:       "web-lite",
 				ResultsPerPage: 10,
@@ -151,6 +156,9 @@ func applyEnvOverrides(cfg *Config) {
 	if v := os.Getenv("BRAVE_ACCESS_TOKEN"); v != "" {
 		cfg.Engines.Brave.AccessToken = v
 	}
+	if v := os.Getenv("YOUTUBE_API_KEY"); v != "" {
+		cfg.Engines.YouTube.APIKey = v
+	}
 	if v := os.Getenv("VALKEY_ADDRESS"); v != "" {
 		cfg.Cache.Address = v
 	}