Bump @clerk/express from 1.7.20 to 2.1.21 in /backend

Bumps [@clerk/express](https://github.com/clerk/javascript/tree/HEAD/packages/express) from 1.7.20 to 2.1.21.
- [Release notes](https://github.com/clerk/javascript/releases)
- [Changelog](https://github.com/clerk/javascript/blob/main/packages/express/CHANGELOG.md)
- [Commits](https://github.com/clerk/javascript/commits/@clerk/express@2.1.21/packages/express)

---
updated-dependencies:
- dependency-name: "@clerk/express"
  dependency-version: 2.1.21
  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

@@ -14,7 +14,7 @@
   "dependencies": {
     "@anthropic-ai/sdk": "^0.61.0",
     "@aws-sdk/client-s3": "^3.879.0",
-    "@clerk/express": "^1.7.20",
+    "@clerk/express": "^2.1.21",
     "@google/generative-ai": "^0.1.3",
     "@polar-sh/express": "^0.3.4",
     "@polar-sh/sdk": "^0.34.10",