feat: 启用豆包二遍识别模式以提升实时性和准确率

- 切换到 bigmodel_async endpoint 并启用 enable_nonstream
- 第一遍流式识别提供实时文字预览
- VAD 分句后自动触发第二遍非流式识别提升准确率
- 修改文本处理逻辑从累加改为替换（适配 full 模式）
- 统一配置字段命名：app_key → app_id, access_key → access_token

internal/config/load.go

@@ -41,11 +41,11 @@ func Load(configPath string) (Config, error) {
 
 // applyEnv overrides config fields with environment variables.
 func applyEnv(cfg *Config) {
-	if v := os.Getenv("DOUBAO_APP_KEY"); v != "" {
-		cfg.Doubao.AppKey = v
+	if v := os.Getenv("DOUBAO_APP_ID"); v != "" {
+		cfg.Doubao.AppID = v
 	}
-	if v := os.Getenv("DOUBAO_ACCESS_KEY"); v != "" {
-		cfg.Doubao.AccessKey = v
+	if v := os.Getenv("DOUBAO_ACCESS_TOKEN"); v != "" {
+		cfg.Doubao.AccessToken = v
 	}
 	if v := os.Getenv("DOUBAO_RESOURCE_ID"); v != "" {
 		cfg.Doubao.ResourceID = v
@@ -62,11 +62,11 @@ func applyEnv(cfg *Config) {
 
 // validate checks required fields.
 func validate(cfg Config) error {
-	if cfg.Doubao.AppKey == "" {
-		return fmt.Errorf("doubao.app_key is required (set DOUBAO_APP_KEY or config.yaml)")
+	if cfg.Doubao.AppID == "" {
+		return fmt.Errorf("doubao.app_id is required (set DOUBAO_APP_ID or config.yaml)")
 	}
-	if cfg.Doubao.AccessKey == "" {
-		return fmt.Errorf("doubao.access_key is required (set DOUBAO_ACCESS_KEY or config.yaml)")
+	if cfg.Doubao.AccessToken == "" {
+		return fmt.Errorf("doubao.access_token is required (set DOUBAO_ACCESS_TOKEN or config.yaml)")
 	}
 	return nil
 }