fix(document): remove @BatchSize from @ManyToOne sender — not supported

Hibernate throws AnnotationException at startup when @BatchSize is placed
on a @ManyToOne field. @BatchSize is only valid on collections (@OneToMany,
@ManyToMany, @ElementCollection). The N+1 for sender is already covered by
the @EntityGraph overrides on DocumentRepository.

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

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

@@ -136,7 +136,6 @@ public class Document {
 
     @ManyToOne(fetch = FetchType.LAZY)
     @JoinColumn(name = "sender_id")
-    @BatchSize(size = 50)
     private Person sender;
 
     @ManyToMany(fetch = FetchType.LAZY)