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>

backend/src/main/resources/application-prod.properties

@@ -0,0 +1,4 @@
+# Database (required)
+spring.datasource.url=${DATABASE_URL}
+spring.datasource.username=${DATABASE_USERNAME}
+spring.datasource.password=${DATABASE_PASSWORD}