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

@@ -56,6 +56,7 @@ func main() {
 		UpstreamURL:   cfg.Upstream.URL,
 		HTTPTimeout:   cfg.HTTPTimeout(),
 		Cache:         searchCache,
+		EnginesConfig: cfg,
 	})
 
 	acSvc := autocomplete.NewService(cfg.Upstream.URL, cfg.HTTPTimeout())

config.example.toml

@@ -22,7 +22,7 @@ url = ""
 [engines]
 # Comma-separated list of engines to execute locally in Go (env: LOCAL_PORTED_ENGINES)
 # Engines not listed here will be proxied to the upstream instance.
-local_ported = ["wikipedia", "arxiv", "crossref", "braveapi", "qwant", "duckduckgo", "github", "reddit", "bing"]
+local_ported = ["wikipedia", "arxiv", "crossref", "braveapi", "qwant", "duckduckgo", "github", "reddit", "bing", "google", "youtube"]
 
 [engines.brave]
 # Brave Search API key (env: BRAVE_API_KEY)
@@ -35,6 +35,10 @@ access_token = ""
 category = "web-lite"
 results_per_page = 10
 
+[engines.youtube]
+# YouTube Data API v3 key (env: YOUTUBE_API_KEY)
+api_key = ""
+
 [cache]
 # Valkey/Redis cache for search results.
 # Leave address empty to disable caching entirely.

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
 	}

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"),
+			apiKey:          braveAPIKey,
-			accessGateToken: os.Getenv("BRAVE_ACCESS_TOKEN"),
+			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",
 		},
 	}

internal/search/service.go

@@ -7,6 +7,7 @@ import (
 	"time"
 
 	"github.com/metamorphosis-dev/kafka/internal/cache"
+	"github.com/metamorphosis-dev/kafka/internal/config"
 	"github.com/metamorphosis-dev/kafka/internal/contracts"
 	"github.com/metamorphosis-dev/kafka/internal/engines"
 	"github.com/metamorphosis-dev/kafka/internal/upstream"
@@ -16,6 +17,7 @@ type ServiceConfig struct {
 	UpstreamURL   string
 	HTTPTimeout   time.Duration
 	Cache         *cache.Cache
+	EnginesConfig *config.Config
 }
 
 type Service struct {
@@ -44,7 +46,7 @@ func NewService(cfg ServiceConfig) *Service {
 	return &Service{
 		upstreamClient: up,
 		planner:        engines.NewPlannerFromEnv(),
-		localEngines:  engines.NewDefaultPortedEngines(httpClient),
+		localEngines:  engines.NewDefaultPortedEngines(httpClient, cfg.EnginesConfig),
 		cache:          cfg.Cache,
 	}
 }