Bump @google/genai from 1.34.0 to 2.16.0 in /backend

Bumps [@google/genai](https://github.com/googleapis/js-genai) from 1.34.0 to 2.16.0.
- [Release notes](https://github.com/googleapis/js-genai/releases)
- [Changelog](https://github.com/googleapis/js-genai/blob/main/CHANGELOG.md)
- [Commits](googleapis/js-genai@v1.34.0...v2.16.0)

---
updated-dependencies:
- dependency-name: "@google/genai"
  dependency-version: 2.16.0
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

backend/package-lock.json

@@ -13,7 +13,7 @@
   "license": "ISC",
   "type": "commonjs",
   "dependencies": {
-    "@google/genai": "^1.34.0",
+    "@google/genai": "^2.16.0",
     "@google/generative-ai": "^0.24.1",
     "cors": "^2.8.5",
     "dotenv": "^17.2.3",