Add check command for anomaly detection

Analyze stored reports and detect:
- DMARC authentication failures with configurable threshold
- TLS-RPT delivery failures
- Report staleness (no reports within --max-age days)

Exit codes for monitoring integration:
  0 = OK, 1 = WARNING, 2 = CRITICAL

Supports --domain, --account, --threshold, --max-age filters
and --format text|json output.

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

Author: linyows