trainingLabels was switched to LAZY fetch in #467 but not added to the
Document.full @NamedEntityGraph. DocumentRepository.findById() uses
Document.full to eagerly load sender/receivers/tags, but the Hibernate
session closes before Jackson serializes the response. Accessing
trainingLabels outside the session throws LazyInitializationException,
causing GET /api/documents/{id} to return HTTP 500.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>