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/resources/application-test.yaml

@@ -0,0 +1,15 @@
+app:
+  s3:
+    endpoint: http://localhost:9000
+    access-key: dummy
+    secret-key: dummy
+    bucket: test-bucket
+    region: us-east-1
+
+spring:
+  datasource:
+    url: will-be-overridden-by-testcontainers
+    username: test
+    password: test
+  mail:
+    host: localhost