fix(duty): add unique keys for duty_day date and slot shift_type

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/scripts/migrate_20260814_duty_schedule.sql

@@ -17,7 +17,7 @@ CREATE TABLE IF NOT EXISTS `duty_day` (
   `del_flag` smallint(6) NOT NULL DEFAULT '0' COMMENT '删除标志',
   PRIMARY KEY (`id`),
   UNIQUE KEY `uk_token` (`access_token`),
-  KEY `idx_tenant_date` (`tenant_no`, `duty_date`, `del_flag`)
+  UNIQUE KEY `uk_tenant_duty_date` (`tenant_no`, `duty_date`, `del_flag`)
 ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COMMENT='值班日（一天一条公开链接）';
 
 CREATE TABLE IF NOT EXISTS `duty_slot` (
@@ -35,6 +35,7 @@ CREATE TABLE IF NOT EXISTS `duty_slot` (
   `update_time` datetime NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP COMMENT '更新时间',
   `del_flag` smallint(6) NOT NULL DEFAULT '0' COMMENT '删除标志',
   PRIMARY KEY (`id`),
+  UNIQUE KEY `uk_day_shift` (`duty_day_id`, `shift_type`, `del_flag`),
   KEY `idx_day` (`duty_day_id`, `del_flag`),
   KEY `idx_tenant` (`tenant_no`)
 ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COMMENT='值班班次名额';