Add Docker Compose setup with PostgreSQL 16 and Spring Boot app

Multi-stage Dockerfile for the backend (build + runtime).
Compose defines db (postgres:16-alpine with healthcheck) and app services.

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

backend/Dockerfile

@@ -0,0 +1,13 @@
+FROM eclipse-temurin:21-jdk-alpine AS build
+WORKDIR /app
+COPY mvnw pom.xml ./
+COPY .mvn .mvn
+RUN ./mvnw dependency:go-offline -B
+COPY src src
+RUN ./mvnw package -DskipTests -B
+
+FROM eclipse-temurin:21-jre-alpine
+WORKDIR /app
+COPY --from=build /app/target/*.jar app.jar
+EXPOSE 8080
+ENTRYPOINT ["java", "-jar", "app.jar"]