feat: add Assignment Prisma model + migration

Links Employee to exactly one of AssetItem/License (XOR enforced at the
Zod layer, not built yet) via nullable FKs, plus assignedAt/returnedAt/
signedAt timestamps. See docs/asset-license-management.md's "Assignment"
section for the full spec.

prisma/migrations/20260803011331_add_assignment/migration.sql

@@ -0,0 +1,22 @@
+-- CreateTable
+CREATE TABLE "assignments" (
+    "id" UUID NOT NULL,
+    "employee_id" UUID NOT NULL,
+    "asset_item_id" UUID,
+    "license_id" UUID,
+    "assigned_at" TIMESTAMPTZ(6) NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    "returned_at" TIMESTAMPTZ(6),
+    "signed_at" TIMESTAMPTZ(6),
+    "created_at" TIMESTAMPTZ(6) NOT NULL DEFAULT CURRENT_TIMESTAMP,
+
+    CONSTRAINT "assignments_pkey" PRIMARY KEY ("id")
+);
+
+-- AddForeignKey
+ALTER TABLE "assignments" ADD CONSTRAINT "assignments_employee_id_fkey" FOREIGN KEY ("employee_id") REFERENCES "employees"("id") ON DELETE RESTRICT ON UPDATE CASCADE;
+
+-- AddForeignKey
+ALTER TABLE "assignments" ADD CONSTRAINT "assignments_asset_item_id_fkey" FOREIGN KEY ("asset_item_id") REFERENCES "asset_items"("id") ON DELETE SET NULL ON UPDATE CASCADE;
+
+-- AddForeignKey
+ALTER TABLE "assignments" ADD CONSTRAINT "assignments_license_id_fkey" FOREIGN KEY ("license_id") REFERENCES "licenses"("id") ON DELETE SET NULL ON UPDATE CASCADE;