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/migrations/20260731075337_add_employee/migration.sql

@@ -0,0 +1,13 @@
+-- CreateTable
+CREATE TABLE "employees" (
+    "id" UUID NOT NULL,
+    "name" TEXT NOT NULL,
+    "department" TEXT NOT NULL,
+    "manager_id" UUID,
+    "created_at" TIMESTAMPTZ(6) NOT NULL DEFAULT CURRENT_TIMESTAMP,
+
+    CONSTRAINT "employees_pkey" PRIMARY KEY ("id")
+);
+
+-- AddForeignKey
+ALTER TABLE "employees" ADD CONSTRAINT "employees_manager_id_fkey" FOREIGN KEY ("manager_id") REFERENCES "employees"("id") ON DELETE SET NULL ON UPDATE CASCADE;