fix(test): repair placeholder tests referencing deleted @bete/shared package

packages/shared dihapus (5802d02), modul pindah ke src/shared/. Update
import placeholder.test.ts (gateway + backend) ke path lokal + aktifkan
tests/ di backend vitest config dengan alias @. Sebelumnya vitest run
gateway selalu gagal; sekarang 60+36 tests pass.

services/backend/vitest.config.ts

@@ -1,10 +1,16 @@
+import { fileURLToPath } from "node:url";
 import { defineConfig } from "vitest/config";
 
 export default defineConfig({
+  resolve: {
+    alias: {
+      "@": fileURLToPath(new URL("./src", import.meta.url)),
+    },
+  },
   test: {
     globals: true,
     environment: "node",
-    include: ["src/**/*.test.ts"],
+    include: ["src/**/*.test.ts", "tests/**/*.test.ts"],
     testTimeout: 15000,
   },
 });