refactor: reduce deployment to one compose file

internal/config/config.go

@@ -50,10 +50,7 @@ func Load() (Config, error) {
 		return Config{}, fmt.Errorf("APP_SHUTDOWN_TIMEOUT: %w", err)
 	}
 
-	keyFile := os.Getenv("APP_MASTER_KEY_FILE")
-	if keyFile == "" {
-		return Config{}, fmt.Errorf("APP_MASTER_KEY_FILE is required (mount a Docker secret there)")
-	}
+	keyFile := value("APP_MASTER_KEY_FILE", "/secrets/master_key")
 
 	cookieSecure := mode == "https"
 	if raw := os.Getenv("APP_COOKIE_SECURE"); raw != "" {