[M2][Epic 2.1] Implement monitors with scheduling and filters

[POWDER-RANGER]

Problem

Build monitor abstraction layer allowing scheduled ingestion runs with keyword/sentiment filters.

Tasks

• Create monitors table (schedule, filters, source associations)
• Implement POST /api/monitors (create)
• Implement PUT /api/monitors/{id}/start (activate)
• Implement PUT /api/monitors/{id}/stop (deactivate)
• Add cron-based scheduler (node-cron or pg-cron)
• Support hourly/daily schedule options
• Apply filters (keywords, sentiment range) during ingestion
• Log monitor execution history

Acceptance Criteria

• ✅ Monitor runs hourly without manual intervention
• ✅ Filters applied correctly (unit tested)
• ✅ Monitor status trackable via dashboard

Priority: P1 - High
Labels: backend, monitoring, M2, P1