fix(#221): resolve inherited color on child tags in document responses

Colors are stored only on root-level tags. DocumentService now calls
TagService.resolveEffectiveColors() before returning search results and
single-document responses, so child tags carry their parent's color when
serialised to JSON. Parent tags are batch-loaded in a single query.

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

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

@@ -14,6 +14,7 @@ import org.springframework.web.server.ResponseStatusException;
 
 import java.util.List;
 import java.util.Optional;
+import java.util.Set;
 import java.util.UUID;
 
 import static org.assertj.core.api.Assertions.assertThat;
@@ -233,6 +234,75 @@ class TagServiceTest {
         assertThat(tree.get(0).children().get(0).id()).isEqualTo(childId);
     }
 
+    // ─── resolveEffectiveColors ───────────────────────────────────────────────
+
+    @Test
+    void resolveEffectiveColors_doesNothing_whenCollectionIsEmpty() {
+        tagService.resolveEffectiveColors(List.of());
+        verifyNoInteractions(tagRepository);
+    }
+
+    @Test
+    void resolveEffectiveColors_doesNothing_whenAllTagsHaveOwnColor() {
+        Tag tag = Tag.builder().id(UUID.randomUUID()).name("Root").color("sage").build();
+
+        tagService.resolveEffectiveColors(List.of(tag));
+
+        verify(tagRepository, never()).findAllById(any());
+        assertThat(tag.getColor()).isEqualTo("sage");
+    }
+
+    @Test
+    void resolveEffectiveColors_doesNothing_whenTagHasNoParentAndNoColor() {
+        Tag tag = Tag.builder().id(UUID.randomUUID()).name("Root").build();
+
+        tagService.resolveEffectiveColors(List.of(tag));
+
+        verify(tagRepository, never()).findAllById(any());
+        assertThat(tag.getColor()).isNull();
+    }
+
+    @Test
+    void resolveEffectiveColors_setsParentColor_onChildTagWithNoColor() {
+        UUID parentId = UUID.randomUUID();
+        Tag parent = Tag.builder().id(parentId).name("Parent").color("sage").build();
+        Tag child = Tag.builder().id(UUID.randomUUID()).name("Child").parentId(parentId).build();
+        when(tagRepository.findAllById(Set.of(parentId))).thenReturn(List.of(parent));
+
+        tagService.resolveEffectiveColors(List.of(child));
+
+        assertThat(child.getColor()).isEqualTo("sage");
+    }
+
+    @Test
+    void resolveEffectiveColors_leavesChildUnchanged_whenParentHasNoColor() {
+        UUID parentId = UUID.randomUUID();
+        Tag parent = Tag.builder().id(parentId).name("Parent").build();
+        Tag child = Tag.builder().id(UUID.randomUUID()).name("Child").parentId(parentId).build();
+        when(tagRepository.findAllById(Set.of(parentId))).thenReturn(List.of(parent));
+
+        tagService.resolveEffectiveColors(List.of(child));
+
+        assertThat(child.getColor()).isNull();
+    }
+
+    @Test
+    void resolveEffectiveColors_batchLoadsParents_inOneQuery() {
+        UUID parentId1 = UUID.randomUUID();
+        UUID parentId2 = UUID.randomUUID();
+        Tag parent1 = Tag.builder().id(parentId1).name("P1").color("sage").build();
+        Tag parent2 = Tag.builder().id(parentId2).name("P2").color("sienna").build();
+        Tag child1 = Tag.builder().id(UUID.randomUUID()).name("C1").parentId(parentId1).build();
+        Tag child2 = Tag.builder().id(UUID.randomUUID()).name("C2").parentId(parentId2).build();
+        when(tagRepository.findAllById(Set.of(parentId1, parentId2))).thenReturn(List.of(parent1, parent2));
+
+        tagService.resolveEffectiveColors(List.of(child1, child2));
+
+        verify(tagRepository, times(1)).findAllById(any());
+        assertThat(child1.getColor()).isEqualTo("sage");
+        assertThat(child2.getColor()).isEqualTo("sienna");
+    }
+
     // ─── delete ───────────────────────────────────────────────────────────────
 
     @Test