test: add mock-based e2e tests

[qj0r9j0vc2]

Summary

• Move existing Docker-based e2e tests to test/e2e-docker/
• Add fast mock-based e2e tests in test/e2e/ (~1 second vs ~5 minutes)
• Implement MockNotifier for in-process testing
• Add test harness for complete server setup

Test Scenarios

• Alert creation (Slack & PagerDuty)
• Alert deduplication
• Alert resolution
• Batch processing
• Severity level handling
• Notifier failure resilience
• Health endpoint

Commands

make test-e2e        # Fast mock-based tests
make test-e2e-docker # Docker-based tests
make test-unit       # Unit tests only