feat: implement proxy input file resolution and storage

- Added `inputFiles.ts` for handling input file resolution, including validation and normalization of file types.
- Introduced `proxyFileStore.ts` for managing file storage, retrieval, and deletion with owner-based isolation.
- Created tests for proxy file storage functionality in `proxyFileStore.test.ts`.
- Developed `proxyInputFileResolver.ts` to resolve input files in OpenAI and response formats.
- Added utility functions for handling reasoning signatures in `reasoningTransport.ts` and corresponding tests.
- Enhanced shared input file transformers in `inputFile.ts` for better file handling and type inference.

docs/client-integration.md

@@ -26,6 +26,7 @@ Metapi 暴露标准 OpenAI / Claude 兼容接口，下游客户端只需配置
 | `/v1/messages` | POST | Claude Messages |
 | `/v1/completions` | POST | OpenAI Completions（Legacy） |
 | `/v1/embeddings` | POST | 向量嵌入 |
+| `/v1/files` | POST / GET / DELETE | OpenAI Files 上传、列举、读取与删除 |
 | `/v1/images/generations` | POST | 图像生成 |
 | `/v1/models` | GET | 模型列表 |
 
@@ -52,6 +53,10 @@ Metapi 暴露标准 OpenAI / Claude 兼容接口，下游客户端只需配置
 | 模型提供商 → OpenAI → API 地址 | `https://your-domain.com` |
 | API Key | `PROXY_TOKEN` |
 
+> 说明：Metapi 现在支持标准 OpenAI `/v1/files` 文件链路。  
+> Cherry Studio、Open WebUI 等客户端如果通过标准 OpenAI Files + `Responses` / Chat 文件块发送 PDF、Markdown、JSON、图片、音频附件，即可经由 Metapi 转发。  
+> 如果某个客户端版本仍使用私有附件协议而不是标准 `/v1/files`，则仍需单独适配。
+
 ### Cursor
 
 | 配置项 | 值 |
@@ -142,7 +147,13 @@ curl -sS https://your-domain.com/v1/chat/completions \
   -H "Content-Type: application/json" \
   -d '{"model":"gpt-4o-mini","messages":[{"role":"user","content":"hi"}]}'
 
-# 3. 测试流式
+# 3. 测试文件上传
+curl -sS https://your-domain.com/v1/files \
+  -H "Authorization: Bearer <PROXY_TOKEN>" \
+  -F "purpose=assistants" \
+  -F "file=@./sample.pdf"
+
+# 4. 测试流式
 curl -sS https://your-domain.com/v1/chat/completions \
   -H "Authorization: Bearer <PROXY_TOKEN>" \
   -H "Content-Type: application/json" \