fix: retrofit License.vendor to vendorId FK + createLicense error handling

License.vendor (free-text string) is now vendorId, a required FK to
Vendor, per docs/asset-license-management.md's "## Vendor (chốt
2026-08-02)" spec. Mirrors the Request.employeeId string->FK precedent:
Zod validates UUID format only, Prisma's own FK constraint (P2003) is
caught at insert time and translated into a clean 400 instead of a raw
500. No License data existed yet, so this is a pure column-type
migration, no backfill needed.

Also converts createLicense() to the {ok,value}|{ok,error} result
pattern used elsewhere (createRequest, createAssetItem) instead of
returning a bare LicenseRecord promise - closes the gap flagged in
CLAUDE.md now that License has its first FK-violation error case to
handle.

prisma/migrations/20260802234029_retrofit_license_vendor_id/migration.sql

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

prisma/schema.prisma

@@ -102,17 +102,20 @@ enum LicenseType {
 
 // Software license inventory — see docs/asset-license-management.md's "Data
 // model (sơ bộ)" table (tên, vendor, tổng seat, seat đã dùng, ngày hết hạn,
-// loại). `vendor` is deliberately a plain String for now, not a separate
-// Vendor entity — no Vendor entity exists yet, same "plain string for now"
-// precedent as Employee.department. `seatsUsed` is a stored counter
-// (defaults to 0 at creation), not a computed value — it will only ever be
-// set to 0 here since Assignment (the thing that would consume seats)
-// doesn't exist yet. `expiryDate` is nullable: perpetual licenses never
-// expire, only subscription licenses usually have one.
+// loại). `vendorId` was a free-text `vendor` String until the "## Vendor
+// (chốt 2026-08-02)" retrofit — same precedent as Request.employeeId
+// (string -> FK once Employee existed). No production `License` data
+// existed at the time of this change, so this was a plain column-type
+// migration, not a backfill. `seatsUsed` is a stored counter (defaults to 0
+// at creation), not a computed value — it will only ever be set to 0 here
+// since Assignment (the thing that would consume seats) doesn't exist yet.
+// `expiryDate` is nullable: perpetual licenses never expire, only
+// subscription licenses usually have one.
 model License {
   id          String      @id @default(uuid()) @db.Uuid
   name        String
-  vendor      String
+  vendorId    String      @map("vendor_id") @db.Uuid
+  vendor      Vendor      @relation(fields: [vendorId], references: [id])
   seatsTotal  Int         @map("seats_total")
   seatsUsed   Int         @default(0) @map("seats_used")
   expiryDate  DateTime?   @map("expiry_date") @db.Date
@@ -126,20 +129,21 @@ model License {
 // (chốt 2026-08-02)". Deliberately independent of AssetModel.manufacturer
 // (OEM, e.g. "Dell") — Vendor is the reseller/distributor actually bought
 // from (e.g. "FPT Trading"), a different concept that isn't always the same
-// company. No FKs on this entity yet: it's the starting point for later
-// FKs from PurchaseOrder/License (License.vendor string -> License.vendorId
-// FK retrofit is noted in the doc as a separate follow-up, not done here).
-// taxCode/notes are nullable — not every vendor record has a known VN tax
-// code or extra notes at creation time.
+// company. `licenses` is the back-relation from the License.vendor ->
+// License.vendorId retrofit (same doc section) — PurchaseOrder is a future
+// FK from this entity, not built yet. taxCode/notes are nullable — not
+// every vendor record has a known VN tax code or extra notes at creation
+// time.
 model Vendor {
-  id           String   @id @default(uuid()) @db.Uuid
+  id           String    @id @default(uuid()) @db.Uuid
   name         String
-  contactEmail String   @map("contact_email")
-  contactPhone String   @map("contact_phone")
+  contactEmail String    @map("contact_email")
+  contactPhone String    @map("contact_phone")
   address      String
-  taxCode      String?  @map("tax_code")
+  taxCode      String?   @map("tax_code")
   notes        String?
-  createdAt    DateTime @default(now()) @map("created_at") @db.Timestamptz(6)
+  createdAt    DateTime  @default(now()) @map("created_at") @db.Timestamptz(6)
+  licenses     License[]
 
   @@map("vendors")
 }