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

@@ -23,7 +23,6 @@ import java.util.Collections;
 import java.util.List;
 import java.util.UUID;
 
-import static org.hamcrest.Matchers.containsString;
 import static org.mockito.ArgumentMatchers.any;
 import static org.mockito.Mockito.when;
 import static org.springframework.test.web.servlet.request.MockMvcRequestBuilders.get;
@@ -150,6 +149,7 @@ 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("$.errors").isEmpty());
     }
 
@@ -163,7 +163,8 @@ class DocumentControllerTest {
         mockMvc.perform(multipart("/api/documents/quick-upload").file(file))
                 .andExpect(status().isOk())
                 .andExpect(jsonPath("$.created").isEmpty())
-                .andExpect(jsonPath("$.errors[0]").value(containsString("report.docx")));
+                .andExpect(jsonPath("$.errors[0].filename").value("report.docx"))
+                .andExpect(jsonPath("$.errors[0].code").value("UNSUPPORTED_FILE_TYPE"));
     }
 
     // ─── GET /api/documents/{id}/versions ────────────────────────────────────

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

@@ -221,7 +221,7 @@ class DocumentServiceTest {
         FileService.UploadResult uploadResult = new FileService.UploadResult("documents/uuid_scan001.pdf", "abc123");
         Document saved = Document.builder().id(UUID.randomUUID()).title("scan001").originalFilename("scan001.pdf").build();
 
-        when(documentRepository.findByOriginalFilename("scan001.pdf")).thenReturn(Optional.empty());
+        when(documentRepository.findFirstByOriginalFilename("scan001.pdf")).thenReturn(Optional.empty());
         when(documentRepository.save(any())).thenReturn(saved);
         when(fileService.uploadFile(any(), any())).thenReturn(uploadResult);
 
@@ -241,7 +241,7 @@ class DocumentServiceTest {
                 .id(UUID.randomUUID()).title("Brief an Oma").originalFilename("scan001.pdf")
                 .status(org.raddatz.familienarchiv.model.DocumentStatus.PLACEHOLDER).build();
 
-        when(documentRepository.findByOriginalFilename("scan001.pdf")).thenReturn(Optional.of(placeholder));
+        when(documentRepository.findFirstByOriginalFilename("scan001.pdf")).thenReturn(Optional.of(placeholder));
         when(documentRepository.save(any())).thenReturn(placeholder);
         when(fileService.uploadFile(any(), any())).thenReturn(uploadResult);