test(document): add LazyInit guard + detail regression tests; prune Document.list graph

Remove trainingLabels from Document.list entity graph now that DocumentListItem
does not touch that association. Integration tests guard against future
LazyInitializationException regressions and confirm Document.full still
loads trainingLabels for the detail endpoint.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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

@@ -31,8 +31,7 @@ import java.util.UUID;
 @NamedEntityGraph(name = "Document.list", attributeNodes = {
         @NamedAttributeNode("sender"),
         @NamedAttributeNode("receivers"),
-        @NamedAttributeNode("tags"),
-        @NamedAttributeNode("trainingLabels")
+        @NamedAttributeNode("tags")
 })
 @Entity
 @Table(name = "documents")