feat(backend): add DocumentRepository finder for thumbnail backfill

Adds findByFilePathIsNotNullAndThumbnailKeyIsNull() used by the
upcoming ThumbnailBackfillService to locate documents that have a
file attached but no thumbnail yet.

Refs #307

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

backend/src/main/java/org/raddatz/familienarchiv/repository/DocumentRepository.java

@@ -46,6 +46,8 @@ public interface DocumentRepository extends JpaRepository<Document, UUID>, JpaSp
 
     List<Document> findByFileHashIsNullAndFilePathIsNotNull();
 
+    List<Document> findByFilePathIsNotNullAndThumbnailKeyIsNull();
+
     @Query("SELECT d.id, d.title FROM Document d WHERE d.id IN :ids")
     List<Object[]> findIdAndTitleByIdIn(@Param("ids") Collection<UUID> ids);