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/common/README.md

@@ -23,7 +23,7 @@
 
 除登录接口外，其他接口都需要携带登录后返回的 `session_key`（见：`docs/common/auth.md`）。
 
-## 上传（七牛直传）
+## 上传（阿里云 OSS 直传）
 
 - `docs/common/upload_qiniu.md`
 
@@ -38,3 +38,7 @@
 ## 自动化部署（非 Docker）
 
 - `docs/common/deploy_ci.md`
+
+## 后端开发规范
+
+- `docs/common/backend_convention.md`（编码风格、模块分层、命名约定、接口规范、新增模块流程等）