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/media/image.go

@@ -0,0 +1,106 @@
+package media
+
+import (
+	"bytes"
+	"fmt"
+	"image"
+	_ "image/gif"
+	_ "image/jpeg"
+	_ "image/png"
+	"io"
+
+	"github.com/chai2010/webp"
+	"github.com/disintegration/imaging"
+)
+
+const (
+	DefaultMaxEdge  = 1920
+	DefaultThumbEdge = 400
+	DefaultQuality  = 80
+)
+
+type ProcessOptions struct {
+	MaxEdge    int
+	ThumbEdge  int
+	WebPQuality float32
+}
+
+type ProcessedImage struct {
+	Main       []byte
+	MainType   string // image/webp
+	MainExt    string // .webp
+	Thumb      []byte
+	ThumbType  string
+	ThumbExt   string
+	Width      int
+	Height     int
+	ThumbWidth int
+	ThumbHeight int
+}
+
+// ProcessImage 解码 → 等比缩放 → WebP 主图 + 缩略图
+func ProcessImage(r io.Reader, opt ProcessOptions) (*ProcessedImage, error) {
+	if opt.MaxEdge <= 0 {
+		opt.MaxEdge = DefaultMaxEdge
+	}
+	if opt.ThumbEdge <= 0 {
+		opt.ThumbEdge = DefaultThumbEdge
+	}
+	if opt.WebPQuality <= 0 || opt.WebPQuality > 100 {
+		opt.WebPQuality = DefaultQuality
+	}
+
+	src, _, err := image.Decode(r)
+	if err != nil {
+		return nil, fmt.Errorf("无法解码图片: %w", err)
+	}
+
+	mainImg := fitWithin(src, opt.MaxEdge)
+	thumbImg := fitWithin(src, opt.ThumbEdge)
+
+	mainBuf := &bytes.Buffer{}
+	if err := webp.Encode(mainBuf, mainImg, &webp.Options{Quality: opt.WebPQuality}); err != nil {
+		return nil, fmt.Errorf("webp 编码失败: %w", err)
+	}
+	thumbBuf := &bytes.Buffer{}
+	if err := webp.Encode(thumbBuf, thumbImg, &webp.Options{Quality: opt.WebPQuality}); err != nil {
+		return nil, fmt.Errorf("缩略图编码失败: %w", err)
+	}
+
+	bMain := mainImg.Bounds()
+	bThumb := thumbImg.Bounds()
+	return &ProcessedImage{
+		Main:        mainBuf.Bytes(),
+		MainType:    "image/webp",
+		MainExt:     ".webp",
+		Thumb:       thumbBuf.Bytes(),
+		ThumbType:   "image/webp",
+		ThumbExt:    ".webp",
+		Width:       bMain.Dx(),
+		Height:      bMain.Dy(),
+		ThumbWidth:  bThumb.Dx(),
+		ThumbHeight: bThumb.Dy(),
+	}, nil
+}
+
+func fitWithin(img image.Image, maxEdge int) image.Image {
+	b := img.Bounds()
+	w, h := b.Dx(), b.Dy()
+	if w <= maxEdge && h <= maxEdge {
+		return img
+	}
+	if w >= h {
+		return imaging.Resize(img, maxEdge, 0, imaging.Lanczos)
+	}
+	return imaging.Resize(img, 0, maxEdge, imaging.Lanczos)
+}
+
+// AllowedUploadMIME 上传阶段允许的源格式
+func AllowedUploadMIME(contentType string) bool {
+	switch contentType {
+	case "image/jpeg", "image/png", "image/gif", "image/webp":
+		return true
+	default:
+		return false
+	}
+}