[Backend] Create cron job monitoring API

[SYLVIANNORUKA]

Description: Implement monitoring of scheduled jobs (event poller, digests, backups) with failure alerting.

Requirements:

• Handle job run recording on start and finish
• Track job duration and success state
• Validate missed-run detection against expected schedule
• Manage alerting on repeated failures
• Provide job run history listing

Acceptance Criteria:

• Run recording is handled
• Duration and success state are tracked
• Missed-run detection is validated
• Alerting on repeated failures works
• Run history listing is provided

Technical Details:

Files: backend/src/modules/monitoring/cron-monitor.service.ts
Libraries: @nestjs/schedule, @nestjs/common