Restore incident baseline status when updates change

[jbrooksuk]

Summary

• add a separate baseline incident status that survives timeline edits and deletions
• restore the baseline status when the last status-bearing update is cleared or removed
• cover the migration, API flow, and incident status sync behavior with regression tests

Testing

• vendor/bin/pest tests/Unit/Actions/Incident/CreateIncidentTest.php tests/Unit/Actions/Incident/UpdateIncidentTest.php tests/Unit/Models/IncidentTest.php tests/Unit/Actions/Update/EditUpdateTest.php tests/Unit/Actions/Update/DeleteUpdateTest.php
• vendor/bin/pest tests/Feature/Api/IncidentUpdateTest.php tests/Feature/Database/IncidentBaselineStatusBackfillTest.php
• composer test:lint