test(tag): pin case-colliding tag resolution on real Postgres (#730)

Mocked TagServiceTest can't prove the two things that actually broke:
that findAllByNameIgnoreCase folds umlauts the way Postgres LOWER() does,
and that saving a document tagged with a case-colliding tag no longer
throws NonUniqueResultException. Testcontainers postgres:16-alpine:

- updateDocument on a doc tagged with the child "weihnachten" succeeds
  and keeps exactly the child tag (not the parent).
- findOrCreate("GLÜCKWÜNSCHE") resolves the Glückwünsche/glückwünsche
  umlaut pair deterministically (lowest id) without throwing — the
  regression catcher a plain-ASCII pair would miss.
- bulk-edit funnels through resolveTags → findOrCreate, guarding a
  future refactor that bypasses it.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/src/test/java/org/raddatz/familienarchiv/document/TagCaseCollisionIntegrationTest.java

@@ -0,0 +1,121 @@
+package org.raddatz.familienarchiv.document;
+
+import org.junit.jupiter.api.Test;
+import org.raddatz.familienarchiv.PostgresContainerConfig;
+import org.raddatz.familienarchiv.tag.Tag;
+import org.raddatz.familienarchiv.tag.TagRepository;
+import org.raddatz.familienarchiv.tag.TagService;
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.boot.test.context.SpringBootTest;
+import org.springframework.context.annotation.Import;
+import org.springframework.test.context.ActiveProfiles;
+import org.springframework.test.context.bean.override.mockito.MockitoBean;
+import org.springframework.transaction.annotation.Transactional;
+import software.amazon.awssdk.services.s3.S3Client;
+
+import java.time.LocalDate;
+import java.util.Comparator;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Set;
+import java.util.UUID;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatCode;
+
+/**
+ * #730 — tag-name resolution against a real Postgres. A mocked repo can't prove the two things that
+ * actually break: that {@code findAllByNameIgnoreCase} folds case the way Postgres {@code LOWER()}
+ * does (critical for umlauts like {@code ü}), and that saving a document tagged with a case-colliding
+ * tag no longer throws {@code NonUniqueResultException}. H2 folds case differently, so this pins the
+ * behaviour on {@code postgres:16-alpine}. The four-branch resolution logic itself is covered faster
+ * by the mocked {@code TagServiceTest}.
+ */
+@SpringBootTest(webEnvironment = SpringBootTest.WebEnvironment.NONE)
+@ActiveProfiles("test")
+@Import(PostgresContainerConfig.class)
+@Transactional
+class TagCaseCollisionIntegrationTest {
+
+    @MockitoBean S3Client s3Client;
+    @Autowired DocumentService documentService;
+    @Autowired DocumentRepository documentRepository;
+    @Autowired TagRepository tagRepository;
+    @Autowired TagService tagService;
+
+    private Tag persistTag(String name, String sourceRef, UUID parentId) {
+        return tagRepository.save(Tag.builder().name(name).sourceRef(sourceRef).parentId(parentId).build());
+    }
+
+    private Document persistDocTaggedWith(Tag tag) {
+        return documentRepository.save(Document.builder()
+                .originalFilename("C-7301")
+                .title("Weihnachtsbrief")
+                .documentDate(LocalDate.of(1928, 1, 1))
+                .metaDatePrecision(DatePrecision.YEAR)
+                .status(DocumentStatus.UPLOADED)
+                .tags(new HashSet<>(Set.of(tag)))
+                .build());
+    }
+
+    @Test
+    void updateDocument_succeedsAndKeepsExactChildTag_whenTaggedWithCaseCollidingChild() throws Exception {
+        Tag parent = persistTag("Weihnachten", "Weihnachten", null);
+        Tag child = persistTag("weihnachten", "Weihnachten/weihnachten", parent.getId());
+        Document doc = persistDocTaggedWith(child);
+
+        DocumentUpdateDTO dto = new DocumentUpdateDTO();
+        dto.setTitle("Weihnachtsbrief");
+        dto.setDocumentDate(LocalDate.of(1930, 1, 1)); // change the date — the field that 500'd on staging
+        dto.setMetaDatePrecision(DatePrecision.YEAR);
+        dto.setTags("weihnachten"); // the edit form round-trips the stored child name
+
+        assertThatCode(() -> documentService.updateDocument(doc.getId(), dto, null, null))
+                .doesNotThrowAnyException();
+
+        Set<Tag> tags = documentRepository.findById(doc.getId()).orElseThrow().getTags();
+        assertThat(tags).hasSize(1);
+        assertThat(tags.iterator().next().getId()).isEqualTo(child.getId()); // child kept, not the parent
+    }
+
+    @Test
+    void findOrCreate_resolvesUmlautCollisionDeterministically_withoutThrow() {
+        // The regression catcher: a plain-ASCII pair would stay green even if Postgres folded ü wrongly.
+        Tag parent = persistTag("Glückwünsche", "Glückwünsche", null);
+        Tag child = persistTag("glückwünsche", "Glückwünsche/glückwünsche", parent.getId());
+
+        // Proof that real Postgres LOWER() folds the umlaut so both rows match case-insensitively.
+        assertThat(tagRepository.findAllByNameIgnoreCase("glückwünsche")).hasSize(2);
+
+        // No exact-case "GLÜCKWÜNSCHE" row exists → resolution falls through to the case-insensitive
+        // branch with two candidates and must pick the lowest id deterministically, never throwing.
+        UUID expected = List.of(parent, child).stream().min(Comparator.comparing(Tag::getId)).orElseThrow().getId();
+        Tag first = tagService.findOrCreate("GLÜCKWÜNSCHE");
+        Tag second = tagService.findOrCreate("GLÜCKWÜNSCHE");
+
+        assertThat(first.getId()).isEqualTo(expected);
+        assertThat(second.getId()).isEqualTo(first.getId());
+    }
+
+    @Test
+    void bulkEdit_resolvesCaseCollidingTagThroughFindOrCreate_withoutThrow() {
+        // Bulk-edit shares resolveTags → findOrCreate; this guards a future refactor that bypasses it.
+        Tag parent = persistTag("Weihnachten", "Weihnachten", null);
+        Tag child = persistTag("weihnachten", "Weihnachten/weihnachten", parent.getId());
+        Document doc = documentRepository.save(Document.builder()
+                .originalFilename("C-7302")
+                .title("Brief")
+                .status(DocumentStatus.UPLOADED)
+                .build());
+
+        DocumentBulkEditDTO dto = new DocumentBulkEditDTO();
+        dto.setTagNames(List.of("weihnachten"));
+
+        assertThatCode(() -> documentService.applyBulkEditToDocument(doc.getId(), dto, null))
+                .doesNotThrowAnyException();
+
+        Set<Tag> tags = documentRepository.findById(doc.getId()).orElseThrow().getTags();
+        assertThat(tags).hasSize(1);
+        assertThat(tags.iterator().next().getId()).isEqualTo(child.getId());
+    }
+}