build(deps): bump @prisma/client from 5.22.0 to 6.4.1 in /backend

Bumps [@prisma/client](https://github.com/prisma/prisma/tree/HEAD/packages/client) from 5.22.0 to 6.4.1.
- [Release notes](https://github.com/prisma/prisma/releases)
- [Commits](https://github.com/prisma/prisma/commits/6.4.1/packages/client)

---
updated-dependencies:
- dependency-name: "@prisma/client"
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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

backend/package.json

@@ -32,7 +32,7 @@
     "@getbrevo/brevo": "^2.2.0",
     "@graphql-tools/schema": "^10.0.9",
     "@graphql-yoga/subscription": "^5.0.1",
-    "@prisma/client": "^5.22.0",
+    "@prisma/client": "^6.4.1",
     "@sentry/node": "^8.37.1",
     "@types/express": "^4.17.21",
     "axios": "^1.7.7",