feat: add Vendor Prisma model + migration

prisma/schema.prisma

@@ -122,6 +122,28 @@ model License {
   @@map("licenses")
 }
 
+// Purchasing counterparty — see docs/asset-license-management.md's "## Vendor
+// (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.
+model Vendor {
+  id           String   @id @default(uuid()) @db.Uuid
+  name         String
+  contactEmail String   @map("contact_email")
+  contactPhone String   @map("contact_phone")
+  address      String
+  taxCode      String?  @map("tax_code")
+  notes        String?
+  createdAt    DateTime @default(now()) @map("created_at") @db.Timestamptz(6)
+
+  @@map("vendors")
+}
+
 // Minimal internal auth accounts — NOT company SSO (deliberately ruled out,
 // see CLAUDE.md / auth decision notes). A handful of trusted IT staff
 // accounts, created by hand via scripts/create-user.ts, no self-registration.