test(tag): explicitly stub the subtree rollup query in getTagTree tests (#698)

Address review nit: the older getTagTree tests relied on Mockito's default
empty-list return for findSubtreeDocumentCountsPerTag. Stub it explicitly so
the two-query contract is self-documenting.

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

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

@@ -199,6 +199,7 @@ class TagServiceTest {
     void getTagTree_returnsEmptyList_whenNoTags() {
         when(tagRepository.findAll()).thenReturn(List.of());
         when(tagRepository.findDocumentCountsPerTag()).thenReturn(List.of());
+        when(tagRepository.findSubtreeDocumentCountsPerTag()).thenReturn(List.of());
 
         assertThat(tagService.getTagTree()).isEmpty();
     }
@@ -213,6 +214,7 @@ class TagServiceTest {
         );
         when(tagRepository.findAll()).thenReturn(tags);
         when(tagRepository.findDocumentCountsPerTag()).thenReturn(List.of());
+        when(tagRepository.findSubtreeDocumentCountsPerTag()).thenReturn(List.of());
 
         var tree = tagService.getTagTree();
 
@@ -228,6 +230,7 @@ class TagServiceTest {
         Tag child = Tag.builder().id(childId).name("Child").parentId(parentId).build();
         when(tagRepository.findAll()).thenReturn(List.of(parent, child));
         when(tagRepository.findDocumentCountsPerTag()).thenReturn(List.of());
+        when(tagRepository.findSubtreeDocumentCountsPerTag()).thenReturn(List.of());
 
         var tree = tagService.getTagTree();
 
@@ -247,6 +250,7 @@ class TagServiceTest {
         Tag child = Tag.builder().id(childId).name("Child").parentId(parentId).build();
         when(tagRepository.findAll()).thenReturn(List.of(parent, child));
         when(tagRepository.findDocumentCountsPerTag()).thenReturn(List.of());
+        when(tagRepository.findSubtreeDocumentCountsPerTag()).thenReturn(List.of());
 
         var tree = tagService.getTagTree();
 
@@ -262,6 +266,7 @@ class TagServiceTest {
         when(countEntry.getCount()).thenReturn(5L);
         when(tagRepository.findAll()).thenReturn(List.of(tag));
         when(tagRepository.findDocumentCountsPerTag()).thenReturn(List.of(countEntry));
+        when(tagRepository.findSubtreeDocumentCountsPerTag()).thenReturn(List.of());
 
         var tree = tagService.getTagTree();
 
@@ -272,6 +277,7 @@ class TagServiceTest {
     void getTagTree_callsFindDocumentCountsPerTag_exactlyOnce() {
         when(tagRepository.findAll()).thenReturn(List.of());
         when(tagRepository.findDocumentCountsPerTag()).thenReturn(List.of());
+        when(tagRepository.findSubtreeDocumentCountsPerTag()).thenReturn(List.of());
 
         tagService.getTagTree();