fix(catalog): guard against null external IDs in manga update pipeline

- MangaUpdateService.update(): skip update when both aniListId and malId
  are null, preventing IllegalStateException for manually-created manga
- MangaUpdateService.fetchExternalMangaData(): return null instead of
  throwing when no external IDs available
- MangaResolutionService.createAndNotifyManga(): only enqueue update
  command when at least one external ID exists, avoiding dead-lettered
  queue messages

src/main/java/com/magamochi/catalog/service/MangaResolutionService.java

@@ -157,12 +157,14 @@ public class MangaResolutionService {
           mangaRepository.save(
               Manga.builder().title(title).aniListId(aniListId).malId(malId).build());
 
+      if (nonNull(aniListId) || nonNull(malId)) {
         try {
           mangaUpdateProducer.sendMangaUpdateCommand(new MangaUpdateCommand(manga.getId()));
         } catch (Exception e) {
           log.error(
               "Failed to send manga update command for manga '{}' (ID: {})", title, manga.getId(), e);
         }
+      }
 
       return manga;
     } catch (DataIntegrityViolationException e) {

src/main/java/com/magamochi/catalog/service/MangaUpdateService.java

@@ -1,5 +1,6 @@
 package com.magamochi.catalog.service;
 
+import static java.util.Objects.isNull;
 import static java.util.Objects.nonNull;
 
 import com.magamochi.catalog.model.dto.MangaDataDTO;
@@ -49,6 +50,12 @@ public class MangaUpdateService {
     log.info("Updating manga with ID {}", mangaId);
 
     var manga = mangaService.find(mangaId);
+
+    if (isNull(manga.getAniListId()) && isNull(manga.getMalId())) {
+      log.info("Skipping update for manga {}: no external IDs available", mangaId);
+      return;
+    }
+
     var mangaData = fetchExternalMangaData(manga);
     applyUpdatesToDatabase(manga, mangaData);
 
@@ -95,8 +102,7 @@ public class MangaUpdateService {
       throw (RuntimeException) fetchException;
     }
 
-    throw new IllegalStateException(
+    return null;
-        "Cannot update manga: No external provider IDs found for Manga " + manga.getId());
   }
 
   private void applyUpdatesToDatabase(Manga manga, MangaDataDTO mangaData) {