docs(backend): document @Transactional(readOnly=true) exception in CLAUDE.md

The convention 'read methods are not annotated' has one exception: methods
that return lazily-initialized entities to callers require readOnly=true to
keep the session open. Documents the rule and links to ADR-022.

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

backend/CLAUDE.md

@@ -97,7 +97,10 @@ public class MyEntity {
 
 - Annotated with `@Service`, `@RequiredArgsConstructor`, optionally `@Slf4j`.
 - Write methods: `@Transactional`.
-- Read methods: no annotation (default non-transactional).
+- Read methods: no annotation (default non-transactional) — **except** when the method returns
+  an entity whose lazy associations must remain accessible to the caller after the method
+  returns. In that case, use `@Transactional(readOnly = true)` to keep the Hibernate session
+  open. Removing this annotation causes `LazyInitializationException` in production. See ADR-022.
 - Cross-domain access goes through the other domain's service, never its repository.
 
 ## Error Handling