Add customer domain Traefik routing

database/migrations/004_instance_custom_domain.sql

@@ -0,0 +1,4 @@
+-- Add an optional customer-owned domain for Traefik routing.
+ALTER TABLE instances
+    ADD COLUMN IF NOT EXISTS custom_domain VARCHAR(253) NULL AFTER name,
+    ADD UNIQUE KEY IF NOT EXISTS uq_instances_custom_domain (custom_domain);