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>

tests/test_api.py

@@ -151,32 +151,8 @@ def test_hairline_gen_female(client):
     assert "image_url" not in d["hairline_images"][0]
 
 
-FEATURES = "/api/v1/face/features"
-
-
-def test_features_success(client, monkeypatch):
-    import face_features as ff
-    monkeypatch.setattr(ff, "analyze_features", lambda *a, **k: {
-        "图片是否有人脸": "有人", "脸型": "鹅蛋脸", "face_shape": "鹅蛋脸",
-        "gender": "女", "gene_style": "少女型"})
-    files = {"image_file": ("frontal.jpg", open(fixture("frontal.jpg"), "rb"), "application/octet-stream")}
-    body = client.post(FEATURES, headers=H, files=files).json()
-    assert body["code"] == 0, body
-    f = json.loads(body["data"]["features"])               # features 是 JSON 字符串
-    assert f["face_shape"] == "鹅蛋脸" and f["gender"] == "女"
-
-
-def test_features_no_face_1001(client, monkeypatch):
-    import face_features as ff
-    monkeypatch.setattr(ff, "analyze_features", lambda *a, **k: {"图片是否有人脸": "没人"})
-    files = {"image_file": ("x.jpg", open(fixture("landscape.jpg"), "rb"), "application/octet-stream")}
-    assert client.post(FEATURES, headers=H, files=files).json()["code"] == 1001
-
-
-def test_features_multi_param_1007(client):
-    files = {"image_file": ("frontal.jpg", open(fixture("frontal.jpg"), "rb"), "application/octet-stream")}
-    r = client.post(FEATURES, headers=H, files=files, data={"image_url": "http://x/y.jpg"})
-    assert r.json()["code"] == 1007
+# 接口4（用户特征）已迁到网关本机实现（直接调豆包），不再在 worker；
+# 其测试随实现一起在网关侧做，worker 这边不再覆盖。
 
 
 def test_success_structure(client):