Fix task duplication: add single/multi-instance support with UI configuration and stop all running instances (#2837)

* Initial plan

* Stop running tasks before starting new scheduled instance

Co-authored-by: whyour <22700758+whyour@users.noreply.github.com>

* Add multi-instance support and fix stop to kill all running instances

- Add allow_multiple_instances field to Crontab model (default: 0 for single instance)
- Add validation for new field in commonCronSchema
- Add getAllPids and killAllTasks utility functions
- Update stop method to kill ALL running instances of a task
- Update runCron to respect allow_multiple_instances config
- Backward compatible: defaults to single instance mode

Co-authored-by: whyour <22700758+whyour@users.noreply.github.com>

* Add UI support for allow_multiple_instances configuration

- Add allow_multiple_instances field to ICrontab interface
- Add instance mode selector in task creation/edit modal
- Add translations for instance mode in Chinese and English
- Default to single instance mode for backward compatibility

Co-authored-by: whyour <22700758+whyour@users.noreply.github.com>

* Add allow_multiple_instances column migration and optimize db.ts

- Add allow_multiple_instances column to Crontabs table migration
- Refactor migration code to use data-driven approach
- Replace 11 individual try-catch blocks with single loop
- Improve code maintainability and readability

Co-authored-by: whyour <22700758+whyour@users.noreply.github.com>

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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: whyour <22700758+whyour@users.noreply.github.com>

src/locales/zh-CN.json

@@ -533,5 +533,10 @@
   "日志名称不能超过100个字符": "日志名称不能超过100个字符",
   "未启用": "未启用",
   "默认为 CPU 个数": "默认为 CPU 个数",
-  "最小是 4": "最小是 4"
+  "最小是 4": "最小是 4",
+  "实例模式": "实例模式",
+  "单实例模式：定时启动新任务前会自动停止旧任务；多实例模式：允许同时运行多个任务实例": "单实例模式：定时启动新任务前会自动停止旧任务；多实例模式：允许同时运行多个任务实例",
+  "请选择实例模式": "请选择实例模式",
+  "单实例": "单实例",
+  "多实例": "多实例"
 }