feat(09-06): keep relation required flags off unbound columns

- Schema JSON still reports required relations for the client
- Save validation only adds required for scalar writable fields

cabana/crud.go

@@ -644,7 +644,9 @@ func mergedRules(cc *CompiledController, model any) map[string]string {
 		return out
 	}
 	for _, field := range cc.Form.Fields {
-		if field.Required {
+		// Relation fields are not writable columns. required stays on the
+		// schema for the client, but it cannot be checked by Fill.
+		if field.Required && scalarFormField(field.Type) {
 			out[field.Name] = mergeRequired(out[field.Name])
 		}
 	}