Update release endpoint fails with 500 for non-existent release

Updating release for any non-existent release code results in a `500 Internal Server Error` instead of a 404.

PUT `/api/releases/NONEXISTENT` – yields:
```json
{
  "type": "about:blank",
  "title": "Internal Server Error",
  "status": 500,
  "detail": "No value present",
  "instance": "/api/releases/NONEXISTENT",
  "timestamp": "2026-03-24T19:34:19.522480Z"
}
```
