test(#119): add Testcontainers @DataJpaTest against real PostgreSQL 16

Adds spring-boot-testcontainers and testcontainers-postgresql deps.
PostgresContainerConfig declares a shared @ServiceConnection container
used by DocumentRepositoryTest, PersonRepositoryTest, and an
ApplicationContextTest smoke test.

Flyway migrations are imported via FlywayConfig and run on every test
execution, verifying the migration chain against a real PostgreSQL 16
container. No H2 is used.

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

backend/src/test/java/org/raddatz/familienarchiv/ApplicationContextTest.java

@@ -0,0 +1,25 @@
+package org.raddatz.familienarchiv;
+
+import org.junit.jupiter.api.Test;
+import org.springframework.boot.test.context.SpringBootTest;
+import org.springframework.boot.testcontainers.service.connection.ServiceConnection;
+import org.springframework.context.annotation.Import;
+import org.springframework.test.context.ActiveProfiles;
+import org.springframework.test.context.bean.override.mockito.MockitoBean;
+import org.testcontainers.containers.PostgreSQLContainer;
+import software.amazon.awssdk.services.s3.S3Client;
+
+@SpringBootTest(webEnvironment = SpringBootTest.WebEnvironment.NONE)
+@ActiveProfiles("test")
+@Import(PostgresContainerConfig.class)
+class ApplicationContextTest {
+
+    @MockitoBean
+    S3Client s3Client;
+
+    @Test
+    void contextLoads() {
+        // verifies that the Spring context starts successfully with all beans wired,
+        // Flyway migrations applied, and no configuration errors
+    }
+}