perf(document): add @BatchSize(50) to sender and trainingLabels

Consistent with the @BatchSize already on receivers and tags. Any lazy
code path not covered by an entity graph will batch-load these associations
instead of issuing one query per document.

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

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

@@ -136,6 +136,7 @@ public class Document {
 
     @ManyToOne(fetch = FetchType.LAZY)
     @JoinColumn(name = "sender_id")
+    @BatchSize(size = 50)
     private Person sender;
 
     @ManyToMany(fetch = FetchType.LAZY)
@@ -148,6 +149,7 @@ public class Document {
     @CollectionTable(name = "document_training_labels", joinColumns = @JoinColumn(name = "document_id"))
     @Column(name = "label")
     @Enumerated(EnumType.STRING)
+    @BatchSize(size = 50)
     @Builder.Default
     private Set<TrainingLabel> trainingLabels = new HashSet<>();