review+docs: 接口4 回收到网关、对齐错误码、合并文档为一份实现说明

代码 review 后的清理：
- 接口4 由网关本机实现，worker app.py 的 /face/features 回退 Mock(保持 worker 无外网依赖)；
  worker requirements 标注 volcengine 改为网关侧；移除 worker 的接口4 测试(随实现挪到网关)
- 网关接口4 业务错误 HTTP 状态统一改 200(与其余接口/worker 约定一致，原为400/503)
- 接口文档：gender 非法码 1004(原误写1008)；修正指向已删文档的链接

文档合并：把各接口技术方案/开发任务书/系统架构/网关任务书 合并成 docs/实现说明.md(简要总览)，
删除原 7 份分散文档，README 收敛为索引(实现说明/接口文档/需求/OFFLINE_ASSETS)。

pytest 44 全绿。

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

gateway/app.py

@@ -256,7 +256,7 @@ async def face_features(
     # 三选一校验
     provided = [x for x in (image_file, image_url, image_base64) if x]
     if len(provided) != 1:
-        return JSONResponse(status_code=400, content={
+        return JSONResponse(status_code=200, content={
             "code": 1007, "message": "图片参数错误：必须且只能传 image_file / image_url / image_base64 其中一个",
             "request_id": f"gw-{_uuid.uuid4().hex[:8]}", "data": None,
         })
@@ -265,7 +265,7 @@ async def face_features(
     if image_file:
         raw = await image_file.read()
         if len(raw) > 1_000_000:
-            return JSONResponse(status_code=400, content={
+            return JSONResponse(status_code=200, content={
                 "code": 1006, "message": "文件超出 1 MB 限制",
                 "request_id": f"gw-{_uuid.uuid4().hex[:8]}", "data": None,
             })
@@ -277,7 +277,7 @@ async def face_features(
         try:
             img_bytes = base64.b64decode(b64)
         except Exception:
-            return JSONResponse(status_code=400, content={
+            return JSONResponse(status_code=200, content={
                 "code": 1008, "message": "图片格式不支持（base64 解码失败）",
                 "request_id": f"gw-{_uuid.uuid4().hex[:8]}", "data": None,
             })
@@ -289,13 +289,13 @@ async def face_features(
         feats = await run_in_threadpool(analyze_features, img_bytes, image_url)
     except Exception as ex:
         logger.exception("接口4 豆包调用失败")
-        return JSONResponse(status_code=503, content={
+        return JSONResponse(status_code=200, content={
             "code": 1007, "message": f"分析服务异常：{ex}",
             "request_id": f"gw-{_uuid.uuid4().hex[:8]}", "data": None,
         })
 
     if not has_face(feats):
-        return JSONResponse(status_code=400, content={
+        return JSONResponse(status_code=200, content={
             "code": 1001, "message": "无法识别人像",
             "request_id": f"gw-{_uuid.uuid4().hex[:8]}", "data": None,
         })