- Introduced `custom_field_force_notify` setting to trigger immediate NOTIFY to a zone's nameservers when its SOA serial number changes and the custom field is set to True.
- Updated README to document the new feature and its configuration.
- Refactored notify handler to include a new job for sending forced NOTIFY messages directly to zone nameservers.
- Enhanced zone signal handling to check for SOA serial changes and trigger forced NOTIFY accordingly.
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