test: add integration tests for TodoController and update test configuration.

backend/pom.xml

@@ -50,6 +50,11 @@
 			<artifactId>postgresql</artifactId>
 			<scope>runtime</scope>
 		</dependency>
+		<dependency>
+			<groupId>com.h2database</groupId>
+			<artifactId>h2</artifactId>
+			<scope>test</scope>
+		</dependency>
 		<dependency>
 			<groupId>org.springframework.boot</groupId>
 			<artifactId>spring-boot-starter-data-jpa-test</artifactId>