fix(import): surface S3 failures + already-exists in skippedFiles, a11y + max-height

- Change importSingleDocument return type from boolean to Optional<String>
  so callers in processRows receive the skip reason on every non-success path.
  S3 upload failures now surface as "S3_UPLOAD_FAILED" and already-imported
  documents as "ALREADY_EXISTS" in the skippedFiles list shown in the admin UI.
- Add two new tests: runImportAsync_addsS3UploadFailed_toSkippedFiles and
  runImportAsync_addsAlreadyExists_toSkippedFiles; update
  importSingleDocument_skips_whenDocumentAlreadyUploadedNotPlaceholder and
  the S3-failure test to assert on the Optional return value.
- Add i18n keys for S3_UPLOAD_FAILED and ALREADY_EXISTS in de/en/es messages.
- Svelte ImportStatusCard: add aria-hidden="true" to SVG chevron, wrap
  conditional warning section in aria-live="polite" div, add max-h-64
  overflow-y-auto to skipped-files <ul> to cap height on large batches.

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

backend/src/main/java/org/raddatz/familienarchiv/importing/MassImportService.java

@@ -69,9 +69,15 @@ public class MassImportService {
             @Schema(requiredMode = Schema.RequiredMode.REQUIRED) List<SkippedFile> skippedFiles,
             LocalDateTime startedAt
     ) {
-        @Schema(requiredMode = Schema.RequiredMode.REQUIRED)
+        // Note: @Schema on a record accessor method is not picked up by SpringDoc; the
+        // "skipped" count is a computed convenience field derived from skippedFiles.size().
         @JsonProperty("skipped")
         public int skipped() { return skippedFiles.size(); }
+
+        /** Defensive-copy constructor — callers cannot mutate the stored list after construction. */
+        public ImportStatus {
+            skippedFiles = List.copyOf(skippedFiles);
+        }
     }
 
     record ProcessResult(int processed, List<SkippedFile> skippedFiles) {}
@@ -304,8 +310,10 @@ public class MassImportService {
                 }
             }
 
-            boolean imported = importSingleDocument(cells, fileOnDisk, filename, index);
-            if (imported) {
+            Optional<String> skipReason = importSingleDocument(cells, fileOnDisk, filename, index);
+            if (skipReason.isPresent()) {
+                skippedFiles.add(new SkippedFile(filename, skipReason.get()));
+            } else {
                 processed++;
             }
         }
@@ -328,12 +336,17 @@ public class MassImportService {
         }
     }
 
+    /**
+     * Imports a single document row.
+     *
+     * @return empty Optional on success; an Optional containing the skip reason on failure/skip.
+     */
     @Transactional
-    protected boolean importSingleDocument(List<String> cells, Optional<File> file, String originalFilename, String index) {
+    protected Optional<String> importSingleDocument(List<String> cells, Optional<File> file, String originalFilename, String index) {
         Optional<Document> existing = documentService.findByOriginalFilename(originalFilename);
         if (existing.isPresent() && existing.get().getStatus() != DocumentStatus.PLACEHOLDER) {
             log.info("Dokument {} existiert bereits, überspringe.", originalFilename);
-            return false;
+            return Optional.of("ALREADY_EXISTS");
         }
 
         String archiveBox    = getCell(cells, colBox);
@@ -369,7 +382,7 @@ public class MassImportService {
                 status = DocumentStatus.UPLOADED;
             } catch (Exception e) {
                 log.error("S3 Upload Fehler für {}", file.get().getName(), e);
-                return false;
+                return Optional.of("S3_UPLOAD_FAILED");
             }
         }
 
@@ -411,7 +424,7 @@ public class MassImportService {
             thumbnailAsyncRunner.dispatchAfterCommit(saved.getId());
         }
         log.info("Importiert{}: {}", file.isEmpty() ? " (nur Metadaten)" : "", originalFilename);
-        return true;
+        return Optional.empty();
     }
 
     // --- Helpers ---

backend/src/test/java/org/raddatz/familienarchiv/importing/MassImportServiceTest.java

@@ -154,9 +154,49 @@ class MassImportServiceTest {
                 .build();
         when(documentService.findByOriginalFilename("doc001.pdf")).thenReturn(Optional.of(existing));
 
-        service.importSingleDocument(minimalCells("doc001.pdf"), Optional.empty(), "doc001.pdf", "doc001");
+        Optional<String> result = service.importSingleDocument(minimalCells("doc001.pdf"), Optional.empty(), "doc001.pdf", "doc001");
 
         verify(documentService, never()).save(any());
+        assertThat(result).isPresent().contains("ALREADY_EXISTS");
+    }
+
+    // ─── importSingleDocument — S3 failure surfaced in skippedFiles ──────────
+
+    @Test
+    void runImportAsync_addsS3UploadFailed_toSkippedFiles_whenS3Throws(@TempDir Path tempDir) throws Exception {
+        byte[] pdfHeader = {0x25, 0x50, 0x44, 0x46, 0x2D}; // %PDF-
+        Files.write(tempDir.resolve("upload_fail.pdf"), pdfHeader);
+        buildMinimalImportXlsx(tempDir, "upload_fail.pdf");
+        ReflectionTestUtils.setField(service, "importDir", tempDir.toString());
+        when(documentService.findByOriginalFilename("upload_fail.pdf")).thenReturn(Optional.empty());
+        doThrow(new RuntimeException("S3 unavailable"))
+                .when(s3Client).putObject(any(PutObjectRequest.class), any(RequestBody.class));
+
+        service.runImportAsync();
+
+        assertThat(service.getStatus().skipped()).isEqualTo(1);
+        assertThat(service.getStatus().skippedFiles())
+                .extracting(MassImportService.SkippedFile::filename, MassImportService.SkippedFile::reason)
+                .containsExactly(org.assertj.core.groups.Tuple.tuple("upload_fail.pdf", "S3_UPLOAD_FAILED"));
+    }
+
+    @Test
+    void runImportAsync_addsAlreadyExists_toSkippedFiles_whenDocumentAlreadyUploaded(@TempDir Path tempDir) throws Exception {
+        buildMinimalImportXlsx(tempDir, "existing.pdf");
+        ReflectionTestUtils.setField(service, "importDir", tempDir.toString());
+        Document existing = Document.builder()
+                .id(UUID.randomUUID())
+                .originalFilename("existing.pdf")
+                .status(DocumentStatus.UPLOADED)
+                .build();
+        when(documentService.findByOriginalFilename("existing.pdf")).thenReturn(Optional.of(existing));
+
+        service.runImportAsync();
+
+        assertThat(service.getStatus().skipped()).isEqualTo(1);
+        assertThat(service.getStatus().skippedFiles())
+                .extracting(MassImportService.SkippedFile::reason)
+                .containsExactly("ALREADY_EXISTS");
     }
 
     // ─── importSingleDocument — create new document (metadata only) ───────────
@@ -208,7 +248,7 @@ class MassImportServiceTest {
     }
 
     @Test
-    void importSingleDocument_returnsEarly_whenS3UploadFails(@TempDir Path tempDir) throws Exception {
+    void importSingleDocument_returnsS3UploadFailed_whenS3UploadFails(@TempDir Path tempDir) throws Exception {
         Path tempFile = tempDir.resolve("fail.pdf");
         Files.write(tempFile, "data".getBytes());
 
@@ -216,10 +256,11 @@ class MassImportServiceTest {
         doThrow(new RuntimeException("S3 error"))
                 .when(s3Client).putObject(any(PutObjectRequest.class), any(RequestBody.class));
 
-        service.importSingleDocument(
+        Optional<String> result = service.importSingleDocument(
                 minimalCells("fail.pdf"), Optional.of(tempFile.toFile()), "fail.pdf", "fail");
 
         verify(documentService, never()).save(any());
+        assertThat(result).isPresent().contains("S3_UPLOAD_FAILED");
     }
 
     // ─── importSingleDocument — sender handling ───────────────────────────────