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

server/internal/storage/s3.go

@@ -0,0 +1,104 @@
+package storage
+
+import (
+	"context"
+	"fmt"
+	"io"
+	"net/url"
+	"strings"
+
+	"github.com/minio/minio-go/v7"
+	"github.com/minio/minio-go/v7/pkg/credentials"
+	"kefu-sys/server/internal/config"
+)
+
+// S3Storage 基于 minio-go 的 S3 兼容实现（MinIO / OSS / COS / AWS S3）
+type S3Storage struct {
+	client *minio.Client
+	bucket string
+	base   string
+}
+
+func NewS3(cfg config.StorageConfig) (*S3Storage, error) {
+	endpoint := strings.TrimPrefix(strings.TrimPrefix(cfg.Endpoint, "https://"), "http://")
+	client, err := minio.New(endpoint, &minio.Options{
+		Creds:  credentials.NewStaticV4(cfg.AccessKey, cfg.SecretKey, ""),
+		Secure: cfg.UseSSL,
+		Region: cfg.Region,
+	})
+	if err != nil {
+		return nil, fmt.Errorf("init s3 client: %w", err)
+	}
+	base := NormalizePublicBase(cfg.PublicBase)
+	if base == "" {
+		scheme := "http"
+		if cfg.UseSSL {
+			scheme = "https"
+		}
+		base = fmt.Sprintf("%s://%s/%s", scheme, endpoint, cfg.Bucket)
+	}
+	return &S3Storage{client: client, bucket: cfg.Bucket, base: base}, nil
+}
+
+func (s *S3Storage) EnsureBucket(ctx context.Context) error {
+	exists, err := s.client.BucketExists(ctx, s.bucket)
+	if err != nil {
+		return err
+	}
+	if !exists {
+		if err := s.client.MakeBucket(ctx, s.bucket, minio.MakeBucketOptions{}); err != nil {
+			return err
+		}
+	}
+	// 开发环境尽量开放公共读；生产用 CDN/桶策略配置
+	policy := fmt.Sprintf(`{
+	  "Version":"2012-10-17",
+	  "Statement":[{
+	    "Effect":"Allow",
+	    "Principal":{"AWS":["*"]},
+	    "Action":["s3:GetObject"],
+	    "Resource":["arn:aws:s3:::%s/*"]
+	  }]
+	}`, s.bucket)
+	_ = s.client.SetBucketPolicy(ctx, s.bucket, policy)
+	return nil
+}
+
+func (s *S3Storage) Put(ctx context.Context, key string, r io.Reader, size int64, contentType string) (string, error) {
+	if contentType == "" {
+		contentType = "application/octet-stream"
+	}
+	_, err := s.client.PutObject(ctx, s.bucket, key, r, size, minio.PutObjectOptions{
+		ContentType: contentType,
+	})
+	if err != nil {
+		return "", err
+	}
+	return s.objectURL(key), nil
+}
+
+func (s *S3Storage) Delete(ctx context.Context, key string) error {
+	return s.client.RemoveObject(ctx, s.bucket, key, minio.RemoveObjectOptions{})
+}
+
+func (s *S3Storage) PublicBase() string { return s.base }
+
+func (s *S3Storage) objectURL(key string) string {
+	// PublicBase 形如 http://localhost:9000/kefu
+	return s.base + "/" + strings.TrimPrefix(key, "/")
+}
+
+// ParseKeyFromURL 从公网 URL 还原 object key（删除时用）
+func (s *S3Storage) ParseKeyFromURL(publicURL string) (string, error) {
+	if !strings.HasPrefix(publicURL, s.base+"/") {
+		return "", fmt.Errorf("url not in bucket")
+	}
+	key := strings.TrimPrefix(publicURL, s.base+"/")
+	if key == "" {
+		return "", fmt.Errorf("empty key")
+	}
+	if u, err := url.PathUnescape(key); err == nil {
+		return u, nil
+	}
+	return key, nil
+}