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/routes/admin_routes.go

@@ -108,7 +108,8 @@ func registerAdminRoutes(
 					marketing.DELETE("/templates/:id", templateHandler.AdminDelete)
 
 					marketing.GET("/stats", downloadHandler.AdminStats)
-					marketing.POST("/upload/qiniu/token", downloadHandler.AdminQiniuToken)
+					marketing.POST("/upload/oss/token", downloadHandler.AdminUploadToken)
+				marketing.POST("/upload", downloadHandler.AdminUploadFile)
 				}
 			}
 		}