feat(tag): add upsertBySourceRef keyed on canonical tag_path

Idempotent tag upsert for the Phase-3 importer (ADR-025). source_ref is
the stable identity (the canonical tag_path); on re-import a
human-renamed tag name is preserved while the parent link is refreshed.

Refs #669

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/src/main/java/org/raddatz/familienarchiv/tag/TagRepository.java

@@ -22,6 +22,9 @@ public interface TagRepository extends JpaRepository<Tag, UUID> {
 
     Optional<Tag> findByNameIgnoreCase(String name);
 
+    // Lookup by the canonical tag_path, used for idempotent canonical re-import (Phase 3).
+    Optional<Tag> findBySourceRef(String sourceRef);
+
     List<Tag> findByNameContainingIgnoreCase(String name);
 
     /**

backend/src/main/java/org/raddatz/familienarchiv/tag/TagService.java

@@ -55,6 +55,26 @@ public class TagService {
                 .orElseGet(() -> tagRepository.save(Tag.builder().name(cleanName).build()));
     }
 
+    /**
+     * Idempotent upsert keyed on {@code sourceRef} (the canonical tag_path) for the
+     * Phase-3 importer (ADR-025). On first import the canonical name and parent are
+     * written; on re-import a human-renamed tag name is preserved (the source_ref is the
+     * stable identity, the name is a human-editable label).
+     */
+    @Transactional
+    public Tag upsertBySourceRef(String sourceRef, String name, UUID parentId) {
+        return tagRepository.findBySourceRef(sourceRef)
+                .map(existing -> {
+                    existing.setParentId(parentId);
+                    return tagRepository.save(existing);
+                })
+                .orElseGet(() -> tagRepository.save(Tag.builder()
+                        .sourceRef(sourceRef)
+                        .name(name)
+                        .parentId(parentId)
+                        .build()));
+    }
+
     @Transactional
     public Tag update(UUID id, TagUpdateDTO dto) {
         Tag tag = getById(id);

backend/src/test/java/org/raddatz/familienarchiv/tag/TagImportUpsertTest.java

@@ -0,0 +1,62 @@
+package org.raddatz.familienarchiv.tag;
+
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.ExtendWith;
+import org.mockito.InjectMocks;
+import org.mockito.Mock;
+import org.mockito.junit.jupiter.MockitoExtension;
+
+import java.util.Optional;
+import java.util.UUID;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.mockito.ArgumentMatchers.any;
+import static org.mockito.ArgumentMatchers.argThat;
+import static org.mockito.Mockito.never;
+import static org.mockito.Mockito.verify;
+import static org.mockito.Mockito.when;
+
+@ExtendWith(MockitoExtension.class)
+class TagImportUpsertTest {
+
+    @Mock TagRepository tagRepository;
+    @InjectMocks TagService tagService;
+
+    @Test
+    void upsertBySourceRef_insertsNewTag_whenSourceRefUnknown() {
+        when(tagRepository.findBySourceRef("Themen/Brautbriefe")).thenReturn(Optional.empty());
+        when(tagRepository.save(any())).thenAnswer(inv -> inv.getArgument(0));
+
+        UUID parentId = UUID.randomUUID();
+        Tag result = tagService.upsertBySourceRef("Themen/Brautbriefe", "Brautbriefe", parentId);
+
+        assertThat(result.getSourceRef()).isEqualTo("Themen/Brautbriefe");
+        assertThat(result.getName()).isEqualTo("Brautbriefe");
+        assertThat(result.getParentId()).isEqualTo(parentId);
+    }
+
+    @Test
+    void upsertBySourceRef_updatesInPlace_whenSourceRefExists() {
+        Tag existing = Tag.builder().id(UUID.randomUUID()).name("Brautbriefe")
+                .sourceRef("Themen/Brautbriefe").build();
+        when(tagRepository.findBySourceRef("Themen/Brautbriefe")).thenReturn(Optional.of(existing));
+        when(tagRepository.save(any())).thenAnswer(inv -> inv.getArgument(0));
+
+        tagService.upsertBySourceRef("Themen/Brautbriefe", "Brautbriefe", null);
+
+        verify(tagRepository).save(argThat(t -> t.getId().equals(existing.getId())));
+        verify(tagRepository, never()).save(argThat(t -> t.getId() == null));
+    }
+
+    @Test
+    void upsertBySourceRef_preservesHumanRenamedTag_onReimport() {
+        Tag humanRenamed = Tag.builder().id(UUID.randomUUID()).name("Verlobungsbriefe")
+                .sourceRef("Themen/Brautbriefe").build();
+        when(tagRepository.findBySourceRef("Themen/Brautbriefe")).thenReturn(Optional.of(humanRenamed));
+        when(tagRepository.save(any())).thenAnswer(inv -> inv.getArgument(0));
+
+        Tag result = tagService.upsertBySourceRef("Themen/Brautbriefe", "Brautbriefe", null);
+
+        assertThat(result.getName()).isEqualTo("Verlobungsbriefe");
+    }
+}