add MySQL container to docker-compose and fix backend container/image name

- Add mysql:8.0 service with health check and named volume for data persistence
- Backend depends_on db with service_healthy condition
- Override SD_DB_HOST=db so backend connects to MySQL container via Docker network
- Rename backend container and image to superdreamfront
- Update .env.example to note "db" host for docker-compose usage

Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

.env.example

@@ -8,7 +8,7 @@ SD_PORT=18000
 
 # Database (MySQL)
 SD_DB_TYPE=mysql
-SD_DB_HOST=localhost
+SD_DB_HOST=localhost   # use "db" when running with docker-compose
 SD_DB_PORT=3306
 SD_DB_USER=root
 SD_DB_PASSWORD=

docker-compose.yml

@@ -1,6 +1,25 @@
 services:
+  db:
+    image: mysql:8.0
+    container_name: superdream-db
+    environment:
+      MYSQL_ROOT_PASSWORD: ${SD_DB_PASSWORD:-superdream}
+      MYSQL_DATABASE: ${SD_DB_NAME:-superdream}
+    ports:
+      - "3306:3306"
+    volumes:
+      - db_data:/var/lib/mysql
+    healthcheck:
+      test: ["CMD", "mysqladmin", "ping", "-h", "localhost", "-u", "root", "-p${SD_DB_PASSWORD:-superdream}"]
+      interval: 10s
+      timeout: 5s
+      retries: 5
+      start_period: 30s
+    restart: unless-stopped
+
   backend:
-    container_name: superdream
+    container_name: superdreamfront
+    image: superdreamfront
     build:
       context: .
       dockerfile: Dockerfile
@@ -10,4 +29,12 @@ services:
       - ./data:/backend/data
     env_file:
       - .env
+    environment:
+      SD_DB_HOST: db
+    depends_on:
+      db:
+        condition: service_healthy
     restart: unless-stopped
+
+volumes:
+  db_data: