Phase 3 Step 2: Implement ScheduleService with cron support

Implement comprehensive schedule management service for automated scans:

New Files:
- web/services/schedule_service.py (470 lines)
  * Complete CRUD operations for schedules
  * Cron expression validation using croniter
  * Next run time calculation
  * Execution history tracking
  * Human-readable relative time formatting

- tests/test_schedule_service.py (671 lines, 40+ tests)
  * Create/get/list/update/delete schedule tests
  * Cron validation and next run calculation tests
  * Pagination and filtering tests
  * Schedule history and serialization tests

Changes:
- requirements-web.txt: Add croniter==2.0.1 dependency
- docs/ai/PHASE3.md: Mark Step 1 complete, Step 2 next

Key Features:
- Validates cron expressions before saving
- Automatically calculates next execution time
- Preserves historical scans when schedules deleted
- Supports pagination and filtering by enabled status
- Provides relative time display (e.g., "in 2 hours")

requirements-web.txt

@@ -21,6 +21,7 @@ marshmallow-sqlalchemy==0.29.0
 
 # Background Jobs & Scheduling
 APScheduler==3.10.4
+croniter==2.0.1
 
 # Email Support (Phase 4)
 Flask-Mail==0.9.1