Add built-in scan scheduling

[jzonthemtn]

Summary

Phinder has no built-in scheduler for recurring scans. Users must set up cron jobs or external orchestration to run scans periodically.

Proposal

Add a built-in scheduler:

• Cron-expression-based scan scheduling
• Configurable per data source
• Run as a long-lived daemon or integrate with system schedulers
• Report scheduling status and next-run times

Acceptance Criteria

• Phinder supports cron-style scan scheduling via built-in scheduler
• Schedule configurable via config file or CLI
• Scan results from each run are logged and queryable
• Missed schedules are handled gracefully (run immediately on next startup)
• Tests cover schedule parsing and execution