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/pom.xml

@@ -65,6 +65,16 @@
 			<groupId>org.springframework.boot</groupId>
 			<artifactId>spring-boot-starter-jetty</artifactId>
 		</dependency>
+		<dependency>
+			<groupId>org.springframework.boot</groupId>
+			<artifactId>spring-boot-testcontainers</artifactId>
+			<scope>test</scope>
+		</dependency>
+		<dependency>
+			<groupId>org.testcontainers</groupId>
+			<artifactId>testcontainers-postgresql</artifactId>
+			<scope>test</scope>
+		</dependency>
 		<dependency>
 			<groupId>org.springframework.boot</groupId>
 			<artifactId>spring-boot-starter-actuator-test</artifactId>