docs(document): add WHY comments to @Transactional(readOnly=true) methods

These annotations deviate from the project convention (read methods are
normally unannotated). The comment explains that the session must stay
open for callers to access lazy-loaded collections post-return, preventing
future developers from removing the annotation as a cleanup.

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

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

@@ -636,7 +636,8 @@ public class DocumentService {
         return saved;
     }
 
-    // 0. Zuletzt aktive Dokumente (sortiert nach updatedAt DESC)
+    // @Transactional(readOnly=true) keeps the Hibernate session open so the
+    // lazy-loaded sender and tags on returned documents remain accessible to callers.
     @Transactional(readOnly = true)
     public List<Document> getRecentActivity(int size) {
         return documentRepository.findAll(
@@ -845,6 +846,8 @@ public class DocumentService {
         documentRepository.save(doc);
     }
 
+    // @Transactional(readOnly=true) keeps the Hibernate session open so the
+    // lazy-loaded tags and receivers on the returned document remain accessible to callers.
     @Transactional(readOnly = true)
     public Document getDocumentById(UUID id) {
         Document doc = documentRepository.findById(id)