实现 MinIO 对象存储图片流水线（可迁移 OSS）

- 新增 S3 兼容 Storage 抽象与 MinIO 实现，compose 启动 MinIO
- 上传接口：校验 → 缩放 → WebP → 主图/缩略图入库
- 消息图片 content 改为对象 URL，拒绝 base64
- 工作台/访客端改为先上传再发消息

docker-compose.dev.yml

@@ -11,5 +11,34 @@ services:
     volumes:
       - pgdata:/var/lib/postgresql/data
 
+  minio:
+    image: minio/minio:latest
+    container_name: kefu-minio
+    command: server /data --console-address ":9001"
+    environment:
+      MINIO_ROOT_USER: minioadmin
+      MINIO_ROOT_PASSWORD: minioadmin
+    ports:
+      - "9000:9000"
+      - "9001:9001"
+    volumes:
+      - miniodata:/data
+
+  # 创建 bucket 并开放下载（开发环境）
+  minio-init:
+    image: minio/mc:latest
+    container_name: kefu-minio-init
+    depends_on:
+      - minio
+    entrypoint: >
+      /bin/sh -c "
+      sleep 3;
+      mc alias set local http://minio:9000 minioadmin minioadmin;
+      mc mb -p local/kefu || true;
+      mc anonymous set download local/kefu;
+      echo 'MinIO bucket kefu ready';
+      "
+
 volumes:
   pgdata:
+  miniodata: