feat: add Docker Compose + TestContainers for PostgreSQL

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/test/java/net/siegeln/cameleer/saas/CameleerSaasApplicationTest.java

@@ -0,0 +1,17 @@
+package net.siegeln.cameleer.saas;
+
+import org.junit.jupiter.api.Test;
+import org.springframework.boot.test.context.SpringBootTest;
+import org.springframework.context.annotation.Import;
+import org.springframework.test.context.ActiveProfiles;
+
+@SpringBootTest
+@Import(TestcontainersConfig.class)
+@ActiveProfiles("test")
+class CameleerSaasApplicationTest {
+
+    @Test
+    void contextLoads() {
+        // Verifies the application context starts successfully
+    }
+}

src/test/java/net/siegeln/cameleer/saas/TestcontainersConfig.java

@@ -0,0 +1,16 @@
+package net.siegeln.cameleer.saas;
+
+import org.springframework.boot.test.context.TestConfiguration;
+import org.springframework.boot.testcontainers.service.connection.ServiceConnection;
+import org.springframework.context.annotation.Bean;
+import org.testcontainers.containers.PostgreSQLContainer;
+
+@TestConfiguration(proxyBeanMethods = false)
+public class TestcontainersConfig {
+
+    @Bean
+    @ServiceConnection
+    PostgreSQLContainer<?> postgresContainer() {
+        return new PostgreSQLContainer<>("postgres:16-alpine");
+    }
+}