feat: add Employee Prisma model + migration

Employee directory entity per docs/asset-license-management.md's data
model table (id, name, department, manager). department stays a plain
String for now (not a separate Department entity). managerId is a
nullable self-relation (EmployeeManager) - top-level employees have
managerId = null.

prisma/schema.prisma

@@ -39,6 +39,23 @@ model Request {
   @@map("requests")
 }
 
+// Employee directory — see docs/asset-license-management.md's "Data model
+// (sơ bộ)" table (id, tên, phòng ban, manager). `department` is deliberately
+// a plain String for now, not a separate Department entity — that's a
+// possible future normalization, not built yet. `managerId` is a nullable
+// self-relation: top-level employees (no manager) have `managerId = null`.
+model Employee {
+  id         String     @id @default(uuid()) @db.Uuid
+  name       String
+  department String
+  managerId  String?    @map("manager_id") @db.Uuid
+  manager    Employee?  @relation("EmployeeManager", fields: [managerId], references: [id])
+  reports    Employee[] @relation("EmployeeManager")
+  createdAt  DateTime   @default(now()) @map("created_at") @db.Timestamptz(6)
+
+  @@map("employees")
+}
+
 // 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.