refactor(music-houduan): 重构 Jackson 配置并优化日期序列化

- 移除 application.yml 中的 Jackson 配置
- 新增 JacksonConfig 类，自定义 LocalDateTime 序列化
- 删除 SingerServiceImpl 中未使用的导入

music-houduan/src/main/java/com/test/musichouduan/config/JacksonConfig.java

@@ -0,0 +1,25 @@
+package com.test.musichouduan.config;
+
+import com.fasterxml.jackson.core.JsonGenerator;
+import com.fasterxml.jackson.databind.JsonSerializer;
+import com.fasterxml.jackson.databind.SerializerProvider;
+import org.springframework.boot.jackson.JsonComponent;
+
+import java.io.IOException;
+import java.time.LocalDateTime;
+import java.time.format.DateTimeFormatter;
+
+@JsonComponent
+public class JacksonConfig {
+
+    private static final DateTimeFormatter FORMATTER = DateTimeFormatter.ofPattern("yyyy-MM-dd");
+
+    public static class LocalDateTimeSerializer extends JsonSerializer<LocalDateTime> {
+        @Override
+        public void serialize(LocalDateTime value, JsonGenerator gen, SerializerProvider serializers) throws IOException {
+            if (value != null) {
+                gen.writeString(FORMATTER.format(value));
+            }
+        }
+    }
+}