Restrict root Jest to backend/__tests__ only — stop scanning frontend

Jest 30 default test discovery was finding frontend/src/**/*.test.js
and __tests__/ files when running from the project root. These need
react-scripts (CRA's Babel config) to parse ESM imports and JSX.
Added jest.roots to confine root-level Jest to backend tests only.
Frontend tests run separately via react-scripts test in test-frontend.

package.json

@@ -25,5 +25,14 @@
   "devDependencies": {
     "fast-check": "^4.8.0",
     "jest": "^30.3.0"
+  },
+  "jest": {
+    "roots": [
+      "<rootDir>/backend/__tests__"
+    ],
+    "testPathIgnorePatterns": [
+      "/node_modules/",
+      "integration"
+    ]
   }
 }