refactor(documents): change DocumentBatchMetadataDTO.tags from String to List<String> tagNames

Replaces comma-delimited String with a proper JSON array field — callers no
longer need to pre-serialise. Service drops the split/trim/filter step and
passes tagNames directly to updateDocumentTags().

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

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

@@ -886,6 +886,30 @@ class DocumentControllerTest {
                 .andExpect(status().isBadRequest());
     }
 
+    @Test
+    @WithMockUser(authorities = "WRITE_ALL")
+    void quickUpload_withMetadata_tagNamesJsonArray_parsedCorrectly() throws Exception {
+        Document doc = Document.builder().id(UUID.randomUUID()).title("brief").originalFilename("brief.pdf").build();
+        when(userService.findByEmail(any())).thenReturn(AppUser.builder().id(UUID.randomUUID()).build());
+
+        org.mockito.ArgumentCaptor<DocumentBatchMetadataDTO> captor =
+                org.mockito.ArgumentCaptor.forClass(DocumentBatchMetadataDTO.class);
+        when(documentService.storeDocumentWithBatchMetadata(any(), captor.capture(), anyInt(), any()))
+                .thenReturn(new DocumentService.StoreResult(doc, true));
+
+        org.springframework.mock.web.MockMultipartFile file =
+                new org.springframework.mock.web.MockMultipartFile("files", "brief.pdf", "application/pdf", new byte[]{1});
+        org.springframework.mock.web.MockMultipartFile metadata =
+                new org.springframework.mock.web.MockMultipartFile("metadata", "metadata", "application/json",
+                        "{\"tagNames\":[\"Briefwechsel\",\"Krieg\"]}".getBytes());
+
+        mockMvc.perform(multipart("/api/documents/quick-upload").file(file).file(metadata))
+                .andExpect(status().isOk());
+
+        org.assertj.core.api.Assertions.assertThat(captor.getValue().getTagNames())
+                .containsExactly("Briefwechsel", "Krieg");
+    }
+
     @Test
     @WithMockUser(authorities = "WRITE_ALL")
     void quickUpload_returns400_whenBatchExceedsCap() throws Exception {