feat(upload): warn on duplicate filename with link to existing document

- storeDocument now returns StoreResult(document, isNew) to distinguish
  new uploads from updates to existing documents
- QuickUploadResult gains an `updated` list alongside `created`
- Frontend shows an amber warning with a "View document" link for duplicates
  instead of silently re-uploading and leaving the user confused

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

backend/src/main/java/org/raddatz/familienarchiv/controller/DocumentController.java

@@ -121,17 +121,18 @@ public class DocumentController {
             "application/pdf", "image/jpeg", "image/png", "image/tiff");
 
     public record UploadError(String filename, String code) {}
-    public record QuickUploadResult(List<Document> created, List<UploadError> errors) {}
+    public record QuickUploadResult(List<Document> created, List<Document> updated, List<UploadError> errors) {}
 
     @PostMapping(value = "/quick-upload", consumes = MediaType.MULTIPART_FORM_DATA_VALUE)
     @RequirePermission(Permission.WRITE_ALL)
     public QuickUploadResult quickUpload(
             @RequestPart(value = "files", required = false) List<MultipartFile> files) {
         List<Document> created = new ArrayList<>();
+        List<Document> updated = new ArrayList<>();
         List<UploadError> errors = new ArrayList<>();
 
         if (files == null || files.isEmpty()) {
-            return new QuickUploadResult(created, errors);
+            return new QuickUploadResult(created, updated, errors);
         }
 
         for (MultipartFile file : files) {
@@ -140,14 +141,19 @@ public class DocumentController {
                 continue;
             }
             try {
-                created.add(documentService.storeDocument(file));
+                DocumentService.StoreResult result = documentService.storeDocument(file);
+                if (result.isNew()) {
+                    created.add(result.document());
+                } else {
+                    updated.add(result.document());
+                }
             } catch (Exception e) {
                 errors.add(new UploadError(file.getOriginalFilename(), "FILE_UPLOAD_FAILED"));
                 log.warn("Quick upload failed for file {}: {}", file.getOriginalFilename(), e.getMessage());
             }
         }
 
-        return new QuickUploadResult(created, errors);
+        return new QuickUploadResult(created, updated, errors);
     }
 
     @GetMapping("/search")

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

@@ -42,18 +42,21 @@ public class DocumentService {
     private final DocumentVersionService documentVersionService;
     private final AnnotationService annotationService;
 
+    public record StoreResult(Document document, boolean isNew) {}
+
     /**
      * Lädt eine Datei hoch.
      * - Prüft, ob ein Eintrag (aus Excel) schon existiert.
-     * - Wenn JA: Aktualisiert Status und verknüpft Datei.
-     * - Wenn NEIN: Erstellt neuen Eintrag (wartet auf Metadaten).
+     * - Wenn JA: Aktualisiert Status und verknüpft Datei — isNew = false.
+     * - Wenn NEIN: Erstellt neuen Eintrag — isNew = true.
      */
     @Transactional
-    public Document storeDocument(MultipartFile file) throws IOException {
+    public StoreResult storeDocument(MultipartFile file) throws IOException {
         String originalFilename = file.getOriginalFilename();
 
         // 1. Check for existing record (findFirst to survive duplicate filenames in the DB)
         Optional<Document> existingDoc = documentRepository.findFirstByOriginalFilename(originalFilename);
+        boolean isNew = existingDoc.isEmpty();
         Document document;
 
         if (existingDoc.isPresent()) {
@@ -77,7 +80,7 @@ public class DocumentService {
             document.setStatus(DocumentStatus.UPLOADED);
         }
 
-        return documentRepository.save(document);
+        return new StoreResult(documentRepository.save(document), isNew);
     }
 
     @Transactional