feat(import): enrich baidu and bytedance release metadata

scripts/import_zhipu_data.go

@@ -32,13 +32,13 @@ type ModelPricing struct {
 	SceneTags       []string
 }
 
-func releaseDateValue(raw string) time.Time {
+func releaseDateValue(raw string) any {
 	if raw == "" {
-		return time.Now()
+		return nil
 	}
 	parsed, err := time.Parse("2006-01-02", raw)
 	if err != nil {
-		return time.Now()
+		return nil
 	}
 	return parsed
 }