Simplify docker-compose: app only, DB is external

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

docker-compose.yml

@@ -1,29 +1,7 @@
 services:
-  db:
-    image: mysql:8.2
-    restart: unless-stopped
-    env_file: .env
-    volumes:
-      - mysql_data:/var/lib/mysql
-      - ./db.sql:/docker-entrypoint-initdb.d/01_schema.sql
-    healthcheck:
-      test: ["CMD", "mysqladmin", "ping", "-h", "127.0.0.1", "--silent"]
-      interval: 10s
-      timeout: 5s
-      retries: 10
-      start_period: 30s
-
   app:
     build: .
     ports:
       - "8720:8720"
     env_file: .env
-    environment:
-      MYSQL_HOST: db      # override localhost → compose service name
-    depends_on:
-      db:
-        condition: service_healthy
     restart: unless-stopped
-
-volumes:
-  mysql_data: