chore(document): address non-blocking review feedback on lazy-fetch PR

- Add @BatchSize(50) fallback comments on findBySenderId / findByReceiversId
- Replace silent size() discard in getRecentActivity test with assertThat isNotEmpty()
- Add ADR-022 reference comment above @JsonIgnoreProperties on Person and Tag
- Document within-open-transaction limitation in DocumentLazyLoadingTest Javadoc

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

backend/src/test/java/org/raddatz/familienarchiv/document/DocumentLazyLoadingTest.java

@@ -28,6 +28,17 @@ import static org.assertj.core.api.Assertions.assertThatCode;
 import static org.mockito.ArgumentMatchers.any;
 import static org.mockito.Mockito.when;
 
+/**
+ * Verifies that lazy-loaded associations on {@link Document} are accessible after a service
+ * method returns — i.e. no {@link org.hibernate.LazyInitializationException} is thrown outside
+ * the Hibernate session that loaded the entity.
+ *
+ * <p><b>Known limitation:</b> calling {@code getDocumentById} (or any other service method) from
+ * within an already-open transaction is not covered here. When an outer transaction is active,
+ * the service's own {@code @Transactional} merges into it and Hibernate keeps the same session
+ * open, so the lazy-init guard behaves differently than in a non-transactional caller. This is a
+ * known constraint of the test setup, not a bug in the production code.
+ */
 @SpringBootTest(webEnvironment = SpringBootTest.WebEnvironment.NONE)
 @ActiveProfiles("test")
 @Import(PostgresContainerConfig.class)
@@ -100,6 +111,7 @@ class DocumentLazyLoadingTest {
         }).doesNotThrowAnyException();
         results.forEach(d -> assertThat(d.getSender()).isNotNull());
         results.forEach(d -> assertThat(d.getSender().getLastName()).isNotNull());
+        results.forEach(d -> assertThat(d.getTags()).isNotEmpty());
     }
 
     @Test