refactor(documents): move batch validation from controller into DocumentService

Validation guards (BATCH_TOO_LARGE, titles > files) are domain rules and
belong in the service where they can be unit-tested without the HTTP layer.
Controller now delegates to documentService.validateBatch().

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

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

@@ -204,12 +204,7 @@ public class DocumentController {
             return new QuickUploadResult(created, updated, errors);
         }
 
-        if (files.size() > 50) {
+        documentService.validateBatch(files.size(), metadata);
-            throw DomainException.badRequest(ErrorCode.BATCH_TOO_LARGE, "Batch exceeds maximum of 50 files per request");
-        }
-        if (metadata != null && metadata.getTitles() != null && metadata.getTitles().size() > files.size()) {
-            throw DomainException.badRequest(ErrorCode.VALIDATION_ERROR, "titles count must not exceed files count");
-        }
 
         UUID actorId = requireUserId(authentication);
         long totalBytes = files.stream().mapToLong(MultipartFile::getSize).sum();

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

@@ -133,6 +133,15 @@ public class DocumentService {
         return new StoreResult(saved, isNew);
     }
 
+    public void validateBatch(int fileCount, DocumentBatchMetadataDTO metadata) {
+        if (fileCount > 50) {
+            throw DomainException.badRequest(ErrorCode.BATCH_TOO_LARGE, "Batch exceeds maximum of 50 files per request");
+        }
+        if (metadata != null && metadata.getTitles() != null && metadata.getTitles().size() > fileCount) {
+            throw DomainException.badRequest(ErrorCode.VALIDATION_ERROR, "titles count must not exceed files count");
+        }
+    }
+
     @Transactional
     public StoreResult storeDocumentWithBatchMetadata(
             MultipartFile file, DocumentBatchMetadataDTO metadata, int fileIndex, UUID actorId) throws IOException {

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

@@ -869,6 +869,10 @@ class DocumentControllerTest {
     @WithMockUser(authorities = "WRITE_ALL")
     void quickUpload_withMetadata_rejects400_whenTitlesSizeExceedsFilesSize() throws Exception {
         when(userService.findByEmail(any())).thenReturn(AppUser.builder().id(UUID.randomUUID()).build());
+        org.mockito.Mockito.doThrow(
+                org.raddatz.familienarchiv.exception.DomainException.badRequest(
+                        org.raddatz.familienarchiv.exception.ErrorCode.VALIDATION_ERROR, "titles count must not exceed files count"))
+                .when(documentService).validateBatch(eq(2), any());
 
         org.springframework.mock.web.MockMultipartFile f1 =
                 new org.springframework.mock.web.MockMultipartFile("files", "a.pdf", "application/pdf", new byte[]{1});
@@ -886,6 +890,10 @@ class DocumentControllerTest {
     @WithMockUser(authorities = "WRITE_ALL")
     void quickUpload_returns400_whenBatchExceedsCap() throws Exception {
         when(userService.findByEmail(any())).thenReturn(AppUser.builder().id(UUID.randomUUID()).build());
+        org.mockito.Mockito.doThrow(
+                org.raddatz.familienarchiv.exception.DomainException.badRequest(
+                        org.raddatz.familienarchiv.exception.ErrorCode.BATCH_TOO_LARGE, "Batch exceeds maximum of 50 files per request"))
+                .when(documentService).validateBatch(eq(51), any());
 
         var builder = multipart("/api/documents/quick-upload");
         for (int i = 0; i < 51; i++) {

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

@@ -1813,4 +1813,29 @@ class DocumentServiceTest {
 
         verify(auditService).logAfterCommit(eq(AuditKind.FILE_UPLOADED), isNull(), eq(id), isNull());
     }
+
+    // ─── validateBatch ───────────────────────────────────────────────────────
+
+    @Test
+    void validateBatch_throwsBatchTooLarge_whenFileCountExceedsCap() {
+        assertThatThrownBy(() -> documentService.validateBatch(51, null))
+                .isInstanceOf(DomainException.class)
+                .hasMessageContaining("50");
+    }
+
+    @Test
+    void validateBatch_doesNotThrow_whenFileCountEqualsCapExactly() {
+        documentService.validateBatch(50, null);
+    }
+
+    @Test
+    void validateBatch_throwsValidationError_whenTitlesSizeExceedsFileCount() {
+        org.raddatz.familienarchiv.dto.DocumentBatchMetadataDTO metadata =
+                new org.raddatz.familienarchiv.dto.DocumentBatchMetadataDTO();
+        metadata.setTitles(java.util.List.of("A", "B", "C"));
+
+        assertThatThrownBy(() -> documentService.validateBatch(2, metadata))
+                .isInstanceOf(DomainException.class)
+                .hasMessageContaining("titles");
+    }
 }