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

docs/marketing/README.md

@@ -124,7 +124,7 @@ web/marketing/
 ## 依赖的公共服务
 
 - **鉴权**：复用 `middleware.AuthMiddleware` + `middleware.RequireUserMiddleware`
-- **七牛上传**：模板图片和用户 Logo 均通过七牛直传，复用 `common/qiniu` 模块
+- **OSS 上传**：模板图片和用户 Logo 均通过阿里云 OSS 直传，复用 `common/upload` 模块
 - **Admin Token**：通过 `X-Admin-Token` 请求头鉴权，Token 来自 `.env` 的 `ADMIN_API_TOKEN`
 
 ## Web 管理后台