fix: retrofit AssetItem.model to modelId FK

AssetItem.model was a free-text string with no referential integrity.
Now a required FK to AssetModel, mirroring the Request.employeeId and
License.vendorId retrofit precedent (no backfill needed, no real
AssetItem data existed).

prisma/migrations/20260803010401_retrofit_asset_item_model_id/migration.sql

@@ -0,0 +1,13 @@
+/*
+  Warnings:
+
+  - You are about to drop the column `model` on the `asset_items` table. All the data in the column will be lost.
+  - Added the required column `model_id` to the `asset_items` table without a default value. This is not possible if the table is not empty.
+
+*/
+-- AlterTable
+ALTER TABLE "asset_items" DROP COLUMN "model",
+ADD COLUMN     "model_id" UUID NOT NULL;
+
+-- AddForeignKey
+ALTER TABLE "asset_items" ADD CONSTRAINT "asset_items_model_id_fkey" FOREIGN KEY ("model_id") REFERENCES "asset_models"("id") ON DELETE RESTRICT ON UPDATE CASCADE;