fix(upload): structured error codes for quick-upload, fix duplicate filename crash

- Switch errors from plain strings to { filename, code } objects so the
  frontend can show translated messages instead of raw exception text
- Add UNSUPPORTED_FILE_TYPE error code end-to-end (Java enum → errors.ts
  → de/en/es messages)
- Fix IncorrectResultSizeDataAccessException when a filename exists more
  than once in the DB: use findFirstByOriginalFilename instead of
  findByOriginalFilename in storeDocument()

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

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

@@ -110,14 +110,15 @@ public class DocumentController {
     private static final Set<String> ALLOWED_CONTENT_TYPES = Set.of(
             "application/pdf", "image/jpeg", "image/png", "image/tiff");
 
-    public record QuickUploadResult(List<Document> created, List<String> errors) {}
+    public record UploadError(String filename, String code) {}
+    public record QuickUploadResult(List<Document> created, 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<String> errors = new ArrayList<>();
+        List<UploadError> errors = new ArrayList<>();
 
         if (files == null || files.isEmpty()) {
             return new QuickUploadResult(created, errors);
@@ -125,13 +126,13 @@ public class DocumentController {
 
         for (MultipartFile file : files) {
             if (!ALLOWED_CONTENT_TYPES.contains(file.getContentType())) {
-                errors.add(file.getOriginalFilename() + ": unsupported file type");
+                errors.add(new UploadError(file.getOriginalFilename(), "UNSUPPORTED_FILE_TYPE"));
                 continue;
             }
             try {
                 created.add(documentService.storeDocument(file));
             } catch (Exception e) {
-                errors.add(file.getOriginalFilename() + ": " + e.getMessage());
+                errors.add(new UploadError(file.getOriginalFilename(), "FILE_UPLOAD_FAILED"));
                 log.warn("Quick upload failed for file {}: {}", file.getOriginalFilename(), e.getMessage());
             }
         }

backend/src/main/java/org/raddatz/familienarchiv/exception/ErrorCode.java

@@ -17,6 +17,8 @@ public enum ErrorCode {
     FILE_NOT_FOUND,
     /** An error occurred while uploading a file to object storage. 500 */
     FILE_UPLOAD_FAILED,
+    /** The uploaded file's content type is not supported (PDF/JPEG/PNG/TIFF only). 400 */
+    UNSUPPORTED_FILE_TYPE,
 
     // --- Users ---
     /** A user with the given ID or username does not exist. 404 */

backend/src/main/java/org/raddatz/familienarchiv/repository/DocumentRepository.java

@@ -21,6 +21,9 @@ public interface DocumentRepository extends JpaRepository<Document, UUID>, JpaSp
     // Wichtig für den Abgleich beim Excel-Import & Datei-Upload
     Optional<Document> findByOriginalFilename(String originalFilename);
 
+    // Wie oben, gibt aber nur das erste Ergebnis zurück — sicher wenn doppelte Dateinamen existieren
+    Optional<Document> findFirstByOriginalFilename(String originalFilename);
+
     // Findet alle Dokumente mit einem bestimmten Status
     // z.B. um alle offenen "PLACEHOLDER" zu finden
     List<Document> findByStatus(DocumentStatus status);

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

@@ -52,8 +52,8 @@ public class DocumentService {
     public Document storeDocument(MultipartFile file) throws IOException {
         String originalFilename = file.getOriginalFilename();
 
-        // 1. Check for existing record
-        Optional<Document> existingDoc = documentRepository.findByOriginalFilename(originalFilename);
+        // 1. Check for existing record (findFirst to survive duplicate filenames in the DB)
+        Optional<Document> existingDoc = documentRepository.findFirstByOriginalFilename(originalFilename);
         Document document;
 
         if (existingDoc.isPresent()) {