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>

docker-compose.yml

@@ -64,11 +64,11 @@ services:
       context: ./backend
       dockerfile: Dockerfile
     container_name: archive-backend
-    command: sleep infinity
     restart: unless-stopped
     volumes:
-      - .:/workspaces/familienarchiv:cached
-      - ./import:/import # Mappt den lokalen Ordner "import-data" auf "/import" im Container
+      - ./backend:/app
+      - ./import:/import
+      - maven_cache:/root/.m2
     depends_on:
       db:
         condition: service_healthy
@@ -78,16 +78,21 @@ services:
       SPRING_DATASOURCE_URL: jdbc:postgresql://db:5432/${POSTGRES_DB}
       SPRING_DATASOURCE_USERNAME: ${POSTGRES_USER}
       SPRING_DATASOURCE_PASSWORD: ${POSTGRES_PASSWORD}
-      # MinIO Konfiguration für Spring Boot (S3)
       S3_ENDPOINT: http://minio:9000
       S3_ACCESS_KEY: ${MINIO_ROOT_USER}
       S3_SECRET_KEY: ${MINIO_ROOT_PASSWORD}
       S3_BUCKET_NAME: ${MINIO_DEFAULT_BUCKETS}
-      S3_REGION: us-east-1 # MinIO Standard
+      S3_REGION: us-east-1
     ports:
       - "${PORT_BACKEND}:8080"
     networks:
       - archive-net
+    healthcheck:
+      test: ["CMD-SHELL", "wget -qO- http://localhost:8080/actuator/health | grep -q UP || exit 1"]
+      interval: 15s
+      timeout: 5s
+      retries: 10
+      start_period: 60s
 
   # --- Frontend: SvelteKit (Dev Server) ---
   frontend:
@@ -102,7 +107,7 @@ services:
       minio:
         condition: service_healthy
       backend:
-        condition: service_started
+        condition: service_healthy
     volumes:
       - ./frontend:/app
       # Keep container's node_modules separate from host to avoid OS binary conflicts
@@ -123,3 +128,4 @@ networks:
 
 volumes:
   frontend_node_modules:
+  maven_cache: