fix: validate extra schedules and localize validation errors

Reject invalid extra_schedules with HTTP 400 before create or update reaches the task service. Preserve omitted, null and empty extras, supported cron syntax, and additional metadata.

Localize missing rules, invalid string and array types, and non-object extra entries through the existing per-request translation middleware. Cover POST/PUT rejection without mutations, dynamic English/Chinese switching, and stable error paths.

Validation: 9 related regression tests passed on current develop; backend build passed. The API validation fix also passed full regression and isolated develop/debian-dev acceptance.

Refs #3099

back/validation/schedule.ts

@@ -20,8 +20,10 @@ export const scheduleSchema = Joi.string()
   .required()
   .custom(validateSchedule)
   .messages({
+    'any.required': '定时规则不能为空',
     'any.invalid': '无效的定时规则',
     'string.empty': '定时规则不能为空',
+    'string.base': '定时规则必须是字符串',
   });
 
 export const commonCronSchema = {
@@ -30,7 +32,27 @@ export const commonCronSchema = {
   schedule: scheduleSchema,
   labels: Joi.array().optional().allow(null),
   sub_id: Joi.number().optional().allow(null),
-  extra_schedules: Joi.array().optional().allow(null),
+  extra_schedules: Joi.array()
+    .items(
+      Joi.object({
+        schedule: Joi.string()
+          .required()
+          .custom((value, helpers) =>
+            isValidCronSchedule(value) ? value : helpers.error('any.invalid'),
+          )
+          .messages({
+            'any.required': '定时规则不能为空',
+            'any.invalid': '无效的定时规则',
+            'string.empty': '定时规则不能为空',
+            'string.base': '定时规则必须是字符串',
+          }),
+      })
+        .unknown(true)
+        .messages({ 'object.base': '额外定时规则必须是对象' }),
+    )
+    .optional()
+    .allow(null)
+    .messages({ 'array.base': '额外定时规则必须是数组' }),
   task_before: Joi.string().optional().allow('').allow(null),
   task_after: Joi.string().optional().allow('').allow(null),
   log_name: Joi.string()