test(document): prove the DB rejects end-before-start; assert persisted end (#678)

Addresses Sara's review concerns:
- Add a negative Testcontainers test: saveAndFlush of a RANGE with end < start
  throws DataIntegrityViolationException, proving chk_meta_date_end_after_start
  actually fires (H2 wouldn't) and exercising the backstop's trigger end-to-end.
  Guards against silent app/DB drift if the service guard ever regresses.
- Tighten updateDocument_acceptsRange_whenEndAfterStart to assert the persisted
  end value, not just that save was called.

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

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

@@ -38,7 +38,10 @@ import java.util.Optional;
 import java.util.Set;
 import java.util.UUID;
 
+import org.springframework.dao.DataIntegrityViolationException;
+
 import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatThrownBy;
 
 @DataJpaTest
 @AutoConfigureTestDatabase(replace = AutoConfigureTestDatabase.Replace.NONE)
@@ -635,6 +638,25 @@ class DocumentRepositoryTest {
         assertThat(found.getMetaDatePrecision()).isEqualTo(DatePrecision.RANGE);
     }
 
+    @Test
+    void save_rejectsRange_whenEndBeforeStart_atDbLevel() {
+        // The app guard normally intercepts this, so the DB CHECK never fires in practice.
+        // Persisting directly proves chk_meta_date_end_after_start actually rejects end < start
+        // (H2 would not) — if the app guard ever regresses, a bad row still can't reach the table,
+        // and this is exactly the violation the GlobalExceptionHandler backstop turns into a 400.
+        Document doc = Document.builder()
+                .title("Verdrehte Spanne")
+                .originalFilename("verdreht.pdf")
+                .status(DocumentStatus.UPLOADED)
+                .documentDate(LocalDate.of(1917, 1, 11))
+                .metaDatePrecision(DatePrecision.RANGE)
+                .metaDateEnd(LocalDate.of(1917, 1, 10))
+                .build();
+
+        assertThatThrownBy(() -> documentRepository.saveAndFlush(doc))
+                .isInstanceOf(DataIntegrityViolationException.class);
+    }
+
     // ─── seeding helpers ─────────────────────────────────────────────────────
 
     private Document uploaded(String title) {

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

@@ -283,6 +283,7 @@ class DocumentServiceTest {
         documentService.updateDocument(id,
                 rangeDto(LocalDate.of(1917, 1, 10), LocalDate.of(1917, 1, 11)), null, null);
 
+        assertThat(doc.getMetaDateEnd()).isEqualTo(LocalDate.of(1917, 1, 11));
         verify(documentRepository, atLeastOnce()).save(any());
     }