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-local.properties.example

@@ -0,0 +1,6 @@
+# Local development database
+# Copy this file to application-local.properties and adjust as needed.
+# Start with: ./mvnw spring-boot:run -Dspring-boot.run.profiles=local
+spring.datasource.url=jdbc:postgresql://localhost:5432/fete
+spring.datasource.username=fete
+spring.datasource.password=fete