fix(stammbaum): JOIN FETCH persons in relationship queries

Both /api/network and /api/persons/{id}/relationships threw
LazyInitializationException when toDTO read Person.getDisplayName():
the read-side service methods aren't @Transactional, so the session
closed before the proxy could initialize.

Eagerly fetch r.person and r.relatedPerson in the two queries used
by these endpoints, keeping the no-@Transactional convention for
read methods.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/src/main/java/org/raddatz/familienarchiv/relationship/PersonRelationshipRepository.java

@@ -16,7 +16,11 @@ public interface PersonRelationshipRepository extends JpaRepository<PersonRelati
      * Bulk fetch for the network endpoint — pulls only edges of the given types.
      * The service filters by family_member afterwards.
      */
-    List<PersonRelationship> findAllByRelationTypeIn(Collection<RelationType> types);
+    @Query("SELECT r FROM PersonRelationship r " +
+           "JOIN FETCH r.person " +
+           "JOIN FETCH r.relatedPerson " +
+           "WHERE r.relationType IN :types")
+    List<PersonRelationship> findAllByRelationTypeIn(@Param("types") Collection<RelationType> types);
 
     /** Used for the circular-PARENT_OF check in {@code addRelationship}. */
     boolean existsByPersonIdAndRelatedPersonIdAndRelationType(
@@ -38,6 +42,8 @@ public interface PersonRelationshipRepository extends JpaRepository<PersonRelati
      * Used by the "direct relationships" listings (person edit, side panel).
      */
     @Query("SELECT r FROM PersonRelationship r " +
+           "JOIN FETCH r.person " +
+           "JOIN FETCH r.relatedPerson " +
            "WHERE r.person.id = :personId OR r.relatedPerson.id = :personId")
     List<PersonRelationship> findAllByPersonIdOrRelatedPersonId(@Param("personId") UUID personId);
 }