feat: rename qiniu to oss, add admin upload proxy with thumbnail, add dev-login

- Rename all QINIU_* config/code/docs to OSS_* to match actual Alibaba Cloud OSS
- Refactor upload module from internal/common/qiniu to internal/common/upload
- Add backend proxy upload endpoint (POST /api/admin/marketing/upload) to avoid CORS
- Auto-generate compressed thumbnail (800px, JPEG 80%) on admin image upload
- Add dev-login endpoint (POST /api/v1/auth/dev-login) for H5 debugging
- Add imageutil package for server-side image resizing

Made-with: Cursor

internal/common/imageutil/resize.go

@@ -0,0 +1,72 @@
+package imageutil
+
+import (
+	"bytes"
+	"fmt"
+	"image"
+	"image/jpeg"
+	"image/png"
+	"io"
+	"strings"
+
+	"golang.org/x/image/draw"
+)
+
+type ResizeOptions struct {
+	MaxWidth int
+	Quality  int // JPEG quality 1-100
+}
+
+func DefaultResizeOptions() ResizeOptions {
+	return ResizeOptions{MaxWidth: 800, Quality: 80}
+}
+
+// GenerateThumbnail reads image data, resizes if wider than MaxWidth, and
+// re-encodes as JPEG. Returns the thumbnail bytes and whether the image
+// was actually resized (false if already small enough but still re-encoded).
+func GenerateThumbnail(data []byte, contentType string, opts ResizeOptions) ([]byte, error) {
+	if opts.MaxWidth <= 0 {
+		opts.MaxWidth = 800
+	}
+	if opts.Quality <= 0 || opts.Quality > 100 {
+		opts.Quality = 80
+	}
+
+	src, err := decodeImage(bytes.NewReader(data), contentType)
+	if err != nil {
+		return nil, fmt.Errorf("decode image: %w", err)
+	}
+
+	bounds := src.Bounds()
+	srcW := bounds.Dx()
+	srcH := bounds.Dy()
+
+	dstW := srcW
+	dstH := srcH
+	if srcW > opts.MaxWidth {
+		dstW = opts.MaxWidth
+		dstH = srcH * opts.MaxWidth / srcW
+	}
+
+	dst := image.NewRGBA(image.Rect(0, 0, dstW, dstH))
+	draw.BiLinear.Scale(dst, dst.Bounds(), src, bounds, draw.Over, nil)
+
+	var buf bytes.Buffer
+	if err := jpeg.Encode(&buf, dst, &jpeg.Options{Quality: opts.Quality}); err != nil {
+		return nil, fmt.Errorf("encode jpeg: %w", err)
+	}
+	return buf.Bytes(), nil
+}
+
+func decodeImage(r io.Reader, contentType string) (image.Image, error) {
+	ct := strings.ToLower(contentType)
+	switch {
+	case strings.Contains(ct, "png"):
+		return png.Decode(r)
+	case strings.Contains(ct, "jpeg"), strings.Contains(ct, "jpg"):
+		return jpeg.Decode(r)
+	default:
+		img, _, err := image.Decode(r)
+		return img, err
+	}
+}