feat(knowledge): 图片上传接口（S3 存储，限 5MB jpg/png/webp）

server-go/internal/service/s3.go

@@ -6,6 +6,7 @@ import (
 	"crypto/sha256"
 	"encoding/hex"
 	"fmt"
+	"io"
 	"net/url"
 	"os"
 	"strings"
@@ -151,6 +152,33 @@ func (s *S3Service) UploadFile(bucket, key, filePath string) error {
 	return err
 }
 
+// EnsureBucket 确保 bucket 存在，不存在则创建（幂等）
+func (s *S3Service) EnsureBucket(bucket string) error {
+	ctx, cancel := context.WithTimeout(context.Background(), 30*time.Second)
+	defer cancel()
+	if _, err := s.client.HeadBucket(ctx, &s3.HeadBucketInput{Bucket: aws.String(bucket)}); err == nil {
+		return nil
+	}
+	_, err := s.client.CreateBucket(ctx, &s3.CreateBucketInput{Bucket: aws.String(bucket)})
+	return err
+}
+
+// UploadImage 上传图片对象（支持指定 Content-Type）
+func (s *S3Service) UploadImage(bucket, key string, body io.Reader, contentType string) error {
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Minute)
+	defer cancel()
+	_, err := s.client.PutObject(ctx, &s3.PutObjectInput{
+		Bucket:      aws.String(bucket),
+		Key:         aws.String(key),
+		Body:        body,
+		ContentType: aws.String(contentType),
+	})
+	return err
+}
+
+// Endpoint 返回 S3 endpoint（用于拼接对象公开 URL）
+func (s *S3Service) Endpoint() string { return s.endpoint }
+
 // sha256Hex 计算 SHA256 十六进制摘要
 func sha256Hex(data string) string {
 	h := sha256.Sum256([]byte(data))