T-4: add JPA, Liquibase, Testcontainers, and deployment docs

Set up development infrastructure for TDD: JPA + Liquibase for
database migrations, Testcontainers for integration tests against
real PostgreSQL, profile-based configuration (prod/local), and
README deployment documentation with docker-compose example.

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

Dockerfile

@@ -21,6 +21,7 @@ FROM eclipse-temurin:25-jre-alpine
 WORKDIR /app
 COPY --from=backend-build /app/backend/target/*.jar app.jar
 EXPOSE 8080
+ENV SPRING_PROFILES_ACTIVE=prod
 HEALTHCHECK --interval=30s --timeout=3s --start-period=10s --retries=3 \
   CMD wget -qO- http://localhost:8080/actuator/health || exit 1
 ENTRYPOINT ["java", "-jar", "app.jar"]