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/shared/i18n.ts

@@ -212,6 +212,9 @@ const messages: Record<string, Record<string, string>> = {
     日志设置为忽略: 'Log set to ignore',
     定时规则不能为空: 'Schedule rule cannot be empty',
     无效的定时规则: 'Invalid schedule rule',
+    定时规则必须是字符串: 'Schedule rule must be a string',
+    额外定时规则必须是对象: 'Extra schedule must be an object',
+    额外定时规则必须是数组: 'Extra schedules must be an array',
     '日志名称只能包含中文、字母、数字、下划线、连字符、点和路径分隔符':
       'Log name can only contain Chinese characters, letters, numbers, underscores, hyphens, dots, and path separators',
     日志名称不能超过100个字符: 'Log name cannot exceed 100 characters',