feat(db): add system_settings table and remove per-user scan settings
- Add system_settings table with id, setting_key, setting_value, description, updated_at
- Insert default settings: scan_frequency_minutes=60, auto_scan_enabled=true
- Remove scan_frequency_minutes and auto_scan_enabled from users table
- Migrate from per-user scan settings to system-wide settings
This change enables centralized scan configuration for all libraries
while removing individual user scan preferences.
Database schema changes require container recreation:
podman compose down -v
podman compose up -d
john-okeefe
e360ec2c30