test(tag): close review-flagged gaps in case-collision coverage (#730)

Two adversarial gaps from PR #733 review:

- Unit: exact-case must win even when its id is NOT the lowest, proving
  exact-case short-circuits before the lowest-id tie-break (a naive
  "lowest id across all CI matches" would pick the wrong row).
- Integration: assert findAllByNameIgnoreCase folds the UPPERCASE
  "GLÜCKWÜNSCHE" — the exact string findOrCreate passes — so the umlaut
  proof matches the resolution path under test, not a lowercase probe.

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

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

@@ -85,7 +85,9 @@ class TagCaseCollisionIntegrationTest {
         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);
+        // Query with the UPPERCASE form findOrCreate actually passes — folding LOWER('GLÜCKWÜNSCHE')
+        // against LOWER(name) is the exact step under test; a lowercase probe wouldn't exercise it.
+        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.

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

@@ -65,6 +65,20 @@ class TagServiceTest {
         verify(tagRepository, never()).save(any());
     }
 
+    @Test
+    void findOrCreate_exactCaseWins_evenWhenItsIdIsNotTheLowest() {
+        // Adversarial guard: exact-case must short-circuit BEFORE the lowest-id rule. Here the exact row
+        // has the higher id, so a naive "always pick lowest id across all CI matches" would pick wrong.
+        Tag exactHigherId = Tag.builder().id(UUID.fromString("00000000-0000-0000-0000-000000000009")).name("geburt").build();
+        when(tagRepository.findByName("geburt")).thenReturn(Optional.of(exactHigherId));
+
+        Tag result = tagService.findOrCreate("geburt");
+
+        assertThat(result).isEqualTo(exactHigherId);
+        verify(tagRepository, never()).findAllByNameIgnoreCase(any()); // exact-case wins without consulting the CI list
+        verify(tagRepository, never()).save(any());
+    }
+
     @Test
     void findOrCreate_usesSingleCaseInsensitiveMatch_whenNoExactCase() {
         // Stored name is "Weihnachten"; a save replays "weihnachten" (no exact-case row) → bind to the