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 ---