feat(search): add TagService.findByNameContaining for NL tag resolution

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

backend/src/main/java/org/raddatz/familienarchiv/tag/TagService.java

@@ -46,6 +46,10 @@ public class TagService {
         return enrichWithRelatives(matched);
     }
 
+    public List<Tag> findByNameContaining(String fragment) {
+        return tagRepository.findByNameContainingIgnoreCase(fragment);
+    }
+
     public Tag getById(UUID id) {
         return tagRepository.findById(id)
                 .orElseThrow(() -> DomainException.notFound(ErrorCode.TAG_NOT_FOUND, "Tag not found: " + id));