Add the data model for structures and prevent them from being embedded in most actors.

module/models/Item/Structure.mjs

@@ -0,0 +1,22 @@
+import { DescribedItemData } from "./DescribedItemData.mjs";
+
+export class StructureItemData extends DescribedItemData {
+	static defineSchema() {
+		const fields = foundry.data.fields;
+		return mergeObject(super.defineSchema(), {
+			one_night: new fields.NumberField({
+				initial: null,
+				nullable: true,
+			}),
+			upkeep: new fields.NumberField({
+				initial: null,
+				nullable: true,
+			}),
+			construction_length: new fields.NumberField({
+				min: 0,
+				initial: 0,
+				nullable: false,
+			}),
+		});
+	};
+};