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/schema.prisma

@@ -97,10 +97,20 @@ enum AssetItemStatus {
 // a formal PO (e.g. historical stock predating the system). Set at
 // intake/creation time only — no update endpoint for this field yet, same
 // precedent as License.vendorId being set-at-creation with no PATCH.
+// `modelId` was a free-text `model` String until the "Cập nhật 2026-08-02
+// (2)" retrofit (docs/asset-license-management.md's "Ràng buộc dữ liệu"
+// bullet) — same precedent as Request.employeeId/License.vendorId (string
+// -> FK once the referenced entity existed). No production `AssetItem` data
+// existed at the time of this change, so this was a plain column-type
+// migration, not a backfill. Required (not nullable) — every AssetItem must
+// belong to a real AssetModel. AssetItem deliberately does NOT carry its own
+// `categoryId` — category is derived via `modelId -> AssetModel.categoryId`
+// to avoid two sources of truth that could drift apart.
 model AssetItem {
   id              String          @id @default(uuid()) @db.Uuid
   serialNumber    String          @unique @map("serial_number")
-  model           String
+  modelId         String          @map("model_id") @db.Uuid
+  model           AssetModel      @relation(fields: [modelId], references: [id])
   purchaseDate    DateTime        @map("purchase_date") @db.Date
   warrantyEndDate DateTime?       @map("warranty_end_date") @db.Date
   status          AssetItemStatus @default(in_stock)
@@ -327,6 +337,9 @@ model AssetModel {
   name         String
   specs        Json          @default("{}")
   createdAt    DateTime      @default(now()) @map("created_at") @db.Timestamptz(6)
+  // Back-relation for the AssetItem.model -> AssetItem.modelId retrofit —
+  // see "Cập nhật 2026-08-02 (2)" in docs/asset-license-management.md.
+  assetItems   AssetItem[]
 
   @@map("asset_models")
 }