feat(#68): fall back to filename as title when createDocument gets no title

When a document is created without an explicit title (null or blank),
the service now derives the title from the uploaded filename using the
same titleFromFilename() logic already used by storeDocument — stripping
the extension for plain names and formatting structured names as
"Firstname Lastname (DD.MM.YYYY)".

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

backend/src/main/java/org/raddatz/familienarchiv/service/DocumentService.java

@@ -108,9 +108,13 @@ public class DocumentService {
                     || (dto.getReceiverIds() != null && !dto.getReceiverIds().isEmpty());
         }
 
+        String titleToUse = (dto.getTitle() != null && !dto.getTitle().isBlank())
+                ? dto.getTitle()
+                : titleFromFilename(filename);
+
         Document doc = Document.builder()
                 .originalFilename(filename)
-                .title(dto.getTitle())
+                .title(titleToUse)
                 .documentDate(dto.getDocumentDate())
                 .location(dto.getLocation())
                 .documentLocation(dto.getDocumentLocation())