feat(relationship): add findAllSpouseEdges() for timeline assembly

Returns all SPOUSE_OF edges with JOIN FETCH on both person sides,
preventing N+1 in TimelineService.assembleDerivedEvents() (REQ-011).
Reuses existing findAllByRelationTypeIn query which already JOIN FETCHes.

Refs #776
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/main/java/org/raddatz/familienarchiv/person/relationship/RelationshipService.java

@@ -86,6 +86,15 @@ public class RelationshipService {
         return new NetworkDTO(nodes, edges);
     }
 
+    /**
+     * Returns all {@code SPOUSE_OF} edges with both person sides JOIN FETCHed.
+     * Used by {@code TimelineService.assembleDerivedEvents()} to build Heirat events
+     * without per-edge N+1 queries.
+     */
+    public List<PersonRelationship> findAllSpouseEdges() {
+        return relationshipRepository.findAllByRelationTypeIn(List.of(RelationType.SPOUSE_OF));
+    }
+
     @Transactional
     public RelationshipDTO addRelationship(UUID personId, CreateRelationshipRequest dto) {
         if (personId.equals(dto.relatedPersonId())) {