feat: auto-start Spring Boot backend via docker-compose

Replace the devcontainer (sleep infinity + VS Code image) with a proper
dev setup:
- Dockerfile: eclipse-temurin:21-jdk-alpine running ./mvnw spring-boot:run
- Source mounted at /app, Maven deps cached in named volume maven_cache
- Healthcheck on /actuator/health so frontend waits until backend is ready
- frontend depends_on backend: service_healthy (was service_started)

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

backend/Dockerfile

@@ -1,8 +1,9 @@
-# Wir nutzen Java 21 (LTS), da Spring Boot 3 das empfiehlt
-FROM mcr.microsoft.com/devcontainers/java:1-21-bullseye
+FROM eclipse-temurin:21-jdk-alpine
 
-# Optional: Zusätzliche OS-Pakete installieren
-# RUN apt-get update && apt-get install -y <package-name>
+WORKDIR /app
 
-# Port für Spring Boot
 EXPOSE 8080
+
+# Source code and mvnw are mounted via docker-compose volume at runtime.
+# Maven dependencies are cached in a named volume (~/.m2).
+CMD ["./mvnw", "spring-boot:run"]