test(bulk-upload): add unit tests for storeDocumentWithBatchMetadata

Covers four behaviours of applyBatchMetadata that had no coverage:
title applied by list index, sender resolved via PersonService,
tags applied via updateDocumentTags, and title left unchanged when
the fileIndex exceeds the titles list length.

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

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

@@ -1814,6 +1814,85 @@ class DocumentServiceTest {
         verify(auditService).logAfterCommit(eq(AuditKind.FILE_UPLOADED), isNull(), eq(id), isNull());
     }
 
+    // ─── storeDocumentWithBatchMetadata ──────────────────────────────────────
+
+    private MockMultipartFile pdfFile(String name) {
+        return new MockMultipartFile("file", name, "application/pdf", new byte[]{1});
+    }
+
+    private void stubStoreDocument(String filename) throws Exception {
+        when(documentRepository.findFirstByOriginalFilename(filename)).thenReturn(Optional.empty());
+        when(fileService.uploadFile(any(), any())).thenReturn(new FileService.UploadResult("key", "hash"));
+    }
+
+    @Test
+    void storeDocumentWithBatchMetadata_appliesTitleByIndex() throws Exception {
+        stubStoreDocument("scan01.pdf");
+        when(documentRepository.save(any())).thenAnswer(inv -> inv.getArgument(0));
+
+        org.raddatz.familienarchiv.dto.DocumentBatchMetadataDTO meta = new org.raddatz.familienarchiv.dto.DocumentBatchMetadataDTO();
+        meta.setTitles(List.of("Erster Brief", "Zweiter Brief"));
+
+        DocumentService.StoreResult result = documentService.storeDocumentWithBatchMetadata(pdfFile("scan01.pdf"), meta, 0, null);
+
+        assertThat(result.document().getTitle()).isEqualTo("Erster Brief");
+    }
+
+    @Test
+    void storeDocumentWithBatchMetadata_resolvesSenderViaPersonService() throws Exception {
+        UUID senderId = UUID.randomUUID();
+        stubStoreDocument("scan02.pdf");
+        when(documentRepository.save(any())).thenAnswer(inv -> inv.getArgument(0));
+        Person sender = Person.builder().id(senderId).firstName("Anna").build();
+        when(personService.getById(senderId)).thenReturn(sender);
+
+        org.raddatz.familienarchiv.dto.DocumentBatchMetadataDTO meta = new org.raddatz.familienarchiv.dto.DocumentBatchMetadataDTO();
+        meta.setSenderId(senderId);
+
+        DocumentService.StoreResult result = documentService.storeDocumentWithBatchMetadata(pdfFile("scan02.pdf"), meta, 0, null);
+
+        assertThat(result.document().getSender()).isEqualTo(sender);
+    }
+
+    @Test
+    void storeDocumentWithBatchMetadata_appliesTagsViaUpdateDocumentTags() throws Exception {
+        UUID docId = UUID.randomUUID();
+        when(documentRepository.findFirstByOriginalFilename("scan03.pdf")).thenReturn(Optional.empty());
+        when(fileService.uploadFile(any(), any())).thenReturn(new FileService.UploadResult("key", "hash"));
+        when(documentRepository.save(any())).thenAnswer(inv -> {
+            Document d = inv.getArgument(0);
+            if (d.getId() == null) d.setId(docId);
+            return d;
+        });
+        when(documentRepository.findById(docId)).thenAnswer(inv -> {
+            Document d = new Document();
+            d.setId(docId);
+            return Optional.of(d);
+        });
+        Tag tag = Tag.builder().id(UUID.randomUUID()).name("Familie").build();
+        when(tagService.findOrCreate("Familie")).thenReturn(tag);
+
+        org.raddatz.familienarchiv.dto.DocumentBatchMetadataDTO meta = new org.raddatz.familienarchiv.dto.DocumentBatchMetadataDTO();
+        meta.setTagNames(List.of("Familie"));
+
+        documentService.storeDocumentWithBatchMetadata(pdfFile("scan03.pdf"), meta, 0, null);
+
+        verify(tagService).findOrCreate("Familie");
+    }
+
+    @Test
+    void storeDocumentWithBatchMetadata_leavesTitle_whenIndexExceedsTitlesList() throws Exception {
+        stubStoreDocument("scan04.pdf");
+        when(documentRepository.save(any())).thenAnswer(inv -> inv.getArgument(0));
+
+        org.raddatz.familienarchiv.dto.DocumentBatchMetadataDTO meta = new org.raddatz.familienarchiv.dto.DocumentBatchMetadataDTO();
+        meta.setTitles(List.of("Only One Title"));
+
+        DocumentService.StoreResult result = documentService.storeDocumentWithBatchMetadata(pdfFile("scan04.pdf"), meta, 5, null);
+
+        assertThat(result.document().getTitle()).isEqualTo("scan04");
+    }
+
     // ─── validateBatch ───────────────────────────────────────────────────────
 
     @Test