feat(music): 支持音乐关联多个歌手

- 修改 Music 和 Singer 实体，使用 @ManyToMany 注解实现多对多关联- 在 MusicServiceImpl 中实现歌手名称字符串的分割和处理
- 删除 singer_music 表，改为在代码中处理关联关系

sql/music_db.sql

@@ -202,17 +202,6 @@ CREATE TABLE `singer`  (
   INDEX `idx_type_id`(`type_id` ASC) USING BTREE
 ) ENGINE = InnoDB AUTO_INCREMENT = 4 CHARACTER SET = utf8mb4 COLLATE = utf8mb4_0900_ai_ci COMMENT = '歌手表' ROW_FORMAT = Dynamic;
 
--- ----------------------------
--- Table structure for singer_music
--- ----------------------------
-DROP TABLE IF EXISTS `singer_music`;
-CREATE TABLE `singer_music`  (
-  `singer_id` bigint NOT NULL,
-  `music_id` bigint NOT NULL,
-  PRIMARY KEY (`singer_id`, `music_id`) USING BTREE,
-  INDEX `FKk0fdv208e2ulhcxd9gge63h6j`(`music_id` ASC) USING BTREE
-) ENGINE = InnoDB CHARACTER SET = utf8mb4 COLLATE = utf8mb4_0900_ai_ci ROW_FORMAT = Dynamic;
-
 -- ----------------------------
 -- Table structure for singer_type
 -- ----------------------------