devops(backend): switch to multi-stage Docker build

Replace runtime mvn spring-boot:run with a proper multi-stage build:
- Stage 1 (builder): compiles JAR with BuildKit cache mount for ~/.m2
- Stage 2 (runtime): eclipse-temurin:21-jre with only the JAR

Removes the backend source volume mount and maven_cache named volume.
Deploy with: docker compose up -d --build

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

docker-compose.yml

@@ -112,9 +112,7 @@ services:
     container_name: archive-backend
     restart: unless-stopped
     volumes:
-      - ./backend:/app
       - ./import:/import
-      - maven_cache:/root/.m2
     depends_on:
       db:
         condition: service_healthy
@@ -191,6 +189,5 @@ networks:
 
 volumes:
   frontend_node_modules:
-  maven_cache:
   ocr_models:
   ocr_cache: