Tombstone response for detached DOI redirection

[yuanzhou]

The GET /doi/redirect/<id> endpoint handles the redirection of DataCite DOIs. It will return 404 when a previously published Dataset or Collection gets deleted from Neo4j.

{
  "error": "404 Not Found: Entity of id: b57f7458986473e4fa57fd418ec9f894 not found in Neo4j"
}

Because published DOIs cannot be deleted from DataCite services, it would cause confusion and disconnection. To properly handle this use case, this endpoint should

• further query the target DOI against DataCite REST API if not found in Neo4j
• return a tombstone page as a 200 response with a proper message "This DOI is no longer attached to any meaningful content"