Add cleanup strategy for old database triggers

[coderabbitai]

Description

Add a lifecycle/retention strategy for old triggers in the DatabaseTriggers collection to prevent unbounded growth.

Proposed Solution

Implement one of the following cleanup strategies:

• Add a TTL (Time To Live) field like expires_at to automatically remove old documents
• Implement periodic purge of CANCELLED/TRIGGERED triggers older than N days
• Add background job to clean up old trigger records

Context

This enhancement was identified during code review of PR #428 which introduces the trigger system. Without proper cleanup, the DatabaseTriggers collection could grow indefinitely as triggers are executed and completed.

Related Issues

• Related to Add MongoDB TTL Index to remove old database objects #432

Backlinks

• PR: Adding triggers to graphs #428
• Comment: Adding triggers to graphs #428 (comment)

Requested by: @NiveditJain