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>

---------

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

back/data/cron.ts

@@ -22,6 +22,7 @@ export class Crontab {
   task_before?: string;
   task_after?: string;
   log_name?: string;
+  allow_multiple_instances?: 1 | 0;
 
   constructor(options: Crontab) {
     this.name = options.name;
@@ -47,6 +48,7 @@ export class Crontab {
     this.task_before = options.task_before;
     this.task_after = options.task_after;
     this.log_name = options.log_name;
+    this.allow_multiple_instances = options.allow_multiple_instances || 0;
   }
 }
 
@@ -87,4 +89,5 @@ export const CrontabModel = sequelize.define<CronInstance>('Crontab', {
   task_before: DataTypes.STRING,
   task_after: DataTypes.STRING,
   log_name: DataTypes.STRING,
+  allow_multiple_instances: DataTypes.NUMBER,
 });