feat(infra): 雲端就緒零影響改動 (compose overlay + healthcheck + env)

- docker-compose.yml：app healthcheck 改用 nc -z 127.0.0.1 9000 驗證 PHP-FPM port
- docker-compose.yml：移除 phpmyadmin、mailpit（開發工具不屬於正式服務）
- compose.override.yml：新增，含 phpmyadmin（8081）、mailpit（8025）port mapping
- .env.example：QUEUE_CONNECTION 預設改 redis；補充 FILESYSTEM_DISK、LOG_STACK、SESSION_DRIVER 雲端注意事項
- docs/STARTUP.md：新增「部署模式」章節，明確區分 VPS 與雲端 -f 指令

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

compose.override.yml

@@ -0,0 +1,27 @@
+services:
+  phpmyadmin:
+    image: phpmyadmin/phpmyadmin
+    container_name: cfdive-phpmyadmin
+    restart: unless-stopped
+    ports:
+      - "127.0.0.1:8081:80"
+    environment:
+      PMA_HOST: db
+      PMA_PORT: 3306
+      PMA_USER: ${DB_USERNAME:-cfdiveuser}
+      PMA_PASSWORD: ${DB_PASSWORD}
+      MYSQL_ROOT_PASSWORD: ${MYSQL_ROOT_PASSWORD}
+    networks:
+      - cfdive-network
+    depends_on:
+      db:
+        condition: service_healthy
+
+  mailpit:
+    image: axllent/mailpit
+    container_name: cfdive-mailpit
+    restart: unless-stopped
+    ports:
+      - "127.0.0.1:8025:8025"
+    networks:
+      - cfdive-network