feat: add comment about PostgresIntegrationTest

Author: antonpampukha

src/test/java/com/itasocialacademy/oitassist/PostgresIntegrationTest.java

@@ -7,6 +7,15 @@
 import org.testcontainers.containers.PostgreSQLContainer;
 import org.testcontainers.utility.DockerImageName;
 
+/**
+ * Deliberately does NOT use {@code @Testcontainers}/{@code @Container}: those
+ * tie the container's stop() to the JUnit lifecycle of whichever test class
+ * triggers it first, which kills the container for every other IT class sharing
+ * this singleton across the same JVM (see: Testcontainers "Singleton
+ * Containers" pattern). The container is started once in this static
+ * initializer and lives for the JVM's lifetime; cleanup is handled by the Ryuk
+ * reaper container at JVM exit, not by JUnit.
+ */
 @SpringBootTest(webEnvironment = SpringBootTest.WebEnvironment.NONE)
 @ActiveProfiles("test")
 @Tag("integration")