fix(document): add trainingLabels to Document.full entity graph (#642)

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>

backend/src/main/java/org/raddatz/familienarchiv/document/Document.java

@@ -25,7 +25,8 @@ import java.util.UUID;
 @NamedEntityGraph(name = "Document.full", attributeNodes = {
         @NamedAttributeNode("sender"),
         @NamedAttributeNode("receivers"),
-        @NamedAttributeNode("tags")
+        @NamedAttributeNode("tags"),
+        @NamedAttributeNode("trainingLabels")
 })
 @NamedEntityGraph(name = "Document.list", attributeNodes = {
         @NamedAttributeNode("sender"),