Create new SystemSettingsHandler for managing system-wide scan settings:
- GetScanSettings: retrieve scan frequency and auto-scan status
- UpdateScanSettings: update scan settings (15-1440 minutes range)
- Admin-only access (no user-specific data)
- Key-value based storage instead of per-user settings
Replaces per-user scan settings with centralized system configuration.
This handler is used by /api/libraries/scan-settings endpoints.