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

backend/src/test/java/org/raddatz/familienarchiv/controller/DocumentControllerTest.java

@@ -167,7 +167,8 @@ class DocumentControllerTest {
     void quickUpload_returns200_withValidPdfFile() throws Exception {
         Document doc = Document.builder()
                 .id(UUID.randomUUID()).title("scan001").originalFilename("scan001.pdf").build();
-        when(documentService.storeDocument(any())).thenReturn(doc);
+        when(documentService.storeDocument(any()))
+                .thenReturn(new DocumentService.StoreResult(doc, true));
 
         org.springframework.mock.web.MockMultipartFile file =
                 new org.springframework.mock.web.MockMultipartFile("files", "scan001.pdf", "application/pdf", new byte[]{1});
@@ -175,7 +176,25 @@ class DocumentControllerTest {
         mockMvc.perform(multipart("/api/documents/quick-upload").file(file))
                 .andExpect(status().isOk())
                 .andExpect(jsonPath("$.created[0].title").value("scan001"))
-                .andExpect(jsonPath("$.errors").isArray())
+                .andExpect(jsonPath("$.updated").isEmpty())
+                .andExpect(jsonPath("$.errors").isEmpty());
+    }
+
+    @Test
+    @WithMockUser(authorities = "WRITE_ALL")
+    void quickUpload_placesDocumentInUpdated_whenFilenameAlreadyExists() throws Exception {
+        Document existing = Document.builder()
+                .id(UUID.randomUUID()).title("Alter Brief").originalFilename("scan001.pdf").build();
+        when(documentService.storeDocument(any()))
+                .thenReturn(new DocumentService.StoreResult(existing, false));
+
+        org.springframework.mock.web.MockMultipartFile file =
+                new org.springframework.mock.web.MockMultipartFile("files", "scan001.pdf", "application/pdf", new byte[]{1});
+
+        mockMvc.perform(multipart("/api/documents/quick-upload").file(file))
+                .andExpect(status().isOk())
+                .andExpect(jsonPath("$.created").isEmpty())
+                .andExpect(jsonPath("$.updated[0].title").value("Alter Brief"))
                 .andExpect(jsonPath("$.errors").isEmpty());
     }
 

backend/src/test/java/org/raddatz/familienarchiv/service/DocumentServiceTest.java

@@ -262,7 +262,7 @@ class DocumentServiceTest {
         FileService.UploadResult uploadResult = new FileService.UploadResult("documents/uuid_scan001.pdf", "abc123");
         Document placeholder = Document.builder()
                 .id(UUID.randomUUID()).title("Brief an Oma").originalFilename("scan001.pdf")
-                .status(org.raddatz.familienarchiv.model.DocumentStatus.PLACEHOLDER).build();
+                .status(DocumentStatus.PLACEHOLDER).build();
 
         when(documentRepository.findFirstByOriginalFilename("scan001.pdf")).thenReturn(Optional.of(placeholder));
         when(documentRepository.save(any())).thenReturn(placeholder);
@@ -273,6 +273,37 @@ class DocumentServiceTest {
         assertThat(placeholder.getTitle()).isEqualTo("Brief an Oma");
     }
 
+    @Test
+    void storeDocument_marksResultAsNew_whenNoExistingDocument() throws Exception {
+        org.springframework.mock.web.MockMultipartFile file =
+                new org.springframework.mock.web.MockMultipartFile("file", "new.pdf", "application/pdf", new byte[]{1});
+        Document saved = Document.builder().id(UUID.randomUUID()).originalFilename("new.pdf").build();
+
+        when(documentRepository.findFirstByOriginalFilename("new.pdf")).thenReturn(Optional.empty());
+        when(documentRepository.save(any())).thenReturn(saved);
+        when(fileService.uploadFile(any(), any())).thenReturn(new FileService.UploadResult("documents/new.pdf", "hash"));
+
+        DocumentService.StoreResult result = documentService.storeDocument(file);
+
+        assertThat(result.isNew()).isTrue();
+    }
+
+    @Test
+    void storeDocument_marksResultAsNotNew_whenDocumentWithSameFilenameExists() throws Exception {
+        org.springframework.mock.web.MockMultipartFile file =
+                new org.springframework.mock.web.MockMultipartFile("file", "existing.pdf", "application/pdf", new byte[]{1});
+        Document existing = Document.builder().id(UUID.randomUUID()).originalFilename("existing.pdf")
+                .status(DocumentStatus.UPLOADED).build();
+
+        when(documentRepository.findFirstByOriginalFilename("existing.pdf")).thenReturn(Optional.of(existing));
+        when(documentRepository.save(any())).thenReturn(existing);
+        when(fileService.uploadFile(any(), any())).thenReturn(new FileService.UploadResult("documents/existing.pdf", "hash"));
+
+        DocumentService.StoreResult result = documentService.storeDocument(file);
+
+        assertThat(result.isNew()).isFalse();
+    }
+
     // ─── backfillFileHashes ───────────────────────────────────────────────────
 
     @Test