perf(document): add @EntityGraph(Document.list) for findAll(Pageable)

getRecentActivity calls findAll(Pageable) — the JpaRepository overload
not covered by the existing Specification variants. Without this override,
sender is loaded N+1 per document. Now applies Document.list graph so
sender and tags are fetched eagerly for every findAll(Pageable) call.

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

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

@@ -34,6 +34,8 @@ public interface DocumentRepository extends JpaRepository<Document, UUID>, JpaSp
     @EntityGraph("Document.list")
     List<Document> findAll(Specification<Document> spec);
 
+    @EntityGraph("Document.list")
+    Page<Document> findAll(Pageable pageable);
 
     // Findet ein Dokument anhand des ursprünglichen Dateinamens
     // Wichtig für den Abgleich beim Excel-Import & Datei-Upload