test: Testcontainers 数据库镜像升级为 postgres:18（ADR-008）

- 验收：JAVA_HOME=jdk17 ./mvnw clean test 全量 37 测试 0 失败，Flyway V1 于 PostgreSQL 18.6 执行无兼容问题

patbond-user/src/test/java/com/patbond/patbond/user/TestcontainersConfiguration.java

@@ -7,7 +7,7 @@ import org.testcontainers.containers.PostgreSQLContainer;
 import org.testcontainers.utility.DockerImageName;
 
 /**
- * Shared Testcontainers setup: a disposable postgres:16 (the production
+ * Shared Testcontainers setup: a disposable postgres:18 (the production
  * target version) wired into the Spring context via @ServiceConnection.
  * Flyway runs the real migrations against it on context startup, so every
  * @SpringBootTest in this module exercises the V1 baseline on a clean
@@ -19,6 +19,6 @@ public class TestcontainersConfiguration {
     @Bean
     @ServiceConnection
     PostgreSQLContainer<?> postgresContainer() {
-        return new PostgreSQLContainer<>(DockerImageName.parse("postgres:16"));
+        return new PostgreSQLContainer<>(DockerImageName.parse("postgres:18"));
     }
 }

patbond-user/src/test/java/com/patbond/patbond/user/persistence/UserPersistenceIntegrationTest.java

@@ -24,7 +24,7 @@ import static org.assertj.core.api.Assertions.assertThatThrownBy;
 
 /**
  * Verifies the Flyway V1 baseline and the real persistence semantics against
- * a clean postgres:16 container: migrations apply, constraints hold, and a
+ * a clean postgres:18 container: migrations apply, constraints hold, and a
  * registered user is durably stored in PostgreSQL (readable over a fresh raw
  * JDBC connection, i.e. independent of any application-process memory — the
  * restart-survival semantics that killed the old ConcurrentHashMap storage).