test(documents): @Transactional rollback in DocumentDensityIntegrationTest (#385)

Replaces @DirtiesContext(AFTER_EACH_TEST_METHOD), which restarted
the full Spring context per test (≈10–15s × 7), with @Transactional
rollback. Each test still sees a clean slate via the spring-test
default rollback, but the context is shared across the class.

Wall time for this class dropped from 35s to 17.87s in local runs.

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

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

@@ -11,9 +11,9 @@ import org.raddatz.familienarchiv.tag.TagOperator;
 import org.springframework.beans.factory.annotation.Autowired;
 import org.springframework.boot.test.context.SpringBootTest;
 import org.springframework.context.annotation.Import;
-import org.springframework.test.annotation.DirtiesContext;
 import org.springframework.test.context.ActiveProfiles;
 import org.springframework.test.context.bean.override.mockito.MockitoBean;
+import org.springframework.transaction.annotation.Transactional;
 import software.amazon.awssdk.services.s3.S3Client;
 
 import java.time.LocalDate;
@@ -34,7 +34,7 @@ import static org.assertj.core.api.Assertions.assertThat;
 @SpringBootTest(webEnvironment = SpringBootTest.WebEnvironment.NONE)
 @ActiveProfiles("test")
 @Import(PostgresContainerConfig.class)
-@DirtiesContext(classMode = DirtiesContext.ClassMode.AFTER_EACH_TEST_METHOD)
+@Transactional
 class DocumentDensityIntegrationTest {
 
     @MockitoBean S3Client s3Client;