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;