feat(接口5): 发际线PNG生成(真实实现，替换Mock)

复用接口2 预览管线：gender 必填 → 该性别全部发际线叠加图(同接口2预览) +
最佳(order=1)发际线曲线的面部中间点坐标。无生发(不调 ComfyUI)。

- hairline/service.py: generate_hairline_pngs——N张发际线叠图 +
  best_center(面部中轴眉心x × order1曲线在该处的y)
- app.py: /hairline/generate 真实实现，新增 gender 必填(非法→1004)，
  返回 hairline_images[].image_base64 + best_hairline_center_point；
  无人脸→1001；重活线程池
- 接口文档/OpenAPI: 接口5 新增 gender 入参 + 输出改 base64(网关改url)
- 测试: test_api 接口5(gender必填/N张/center点)，44全绿

实测(5090): female 5张叠图 + center{x,y}，~2.5s(无Flux)。

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

hairline/service.py

@@ -16,7 +16,7 @@ from .face_landmarks import FaceLandmarker
 from .face_parsing import FaceParser
 from .hairline_2d import sample_hairline, smooth_hairline
 from .lift_3d import lift_hairline_to_3d, build_middle_row, assemble_full
-from .render import load_ext_mesh, load_texture_rgba, render_hairline_overlay
+from .render import load_ext_mesh, load_texture_rgba, render_hairline_overlay, build_overlay_layer
 from .mask import build_inpaint_mask, compose_comfy_rgba, mask_from_curve
 from .marker_detect import detect_marker_hairline, path_to_curve_mask
 
@@ -161,6 +161,38 @@ def generate_grow_results(image_bgr: np.ndarray, gender: str):
     return results
 
 
+def generate_hairline_pngs(image_bgr: np.ndarray, gender: str):
+    """接口5：该性别全部发际线叠图(同接口2预览) + 最佳(order1)发际线曲线的面部中间点。
+
+    Returns: {"images":[{hairline_type,order,image_bgr}], "best_center":(x,y)}；无人脸 None。
+    """
+    if gender not in ("male", "female"):
+        raise ValueError(f"gender 必须是 male/female，收到 {gender!r}")
+    ctx = extract_context(image_bgr)
+    if ctx is None:
+        return None
+    h, w = image_bgr.shape[:2]
+    uv, ext_faces = load_ext_mesh()
+    lm = ctx["landmarks"]
+    # 面部中轴 x = 眉心(9/151 中点)
+    face_cx = float((lm[9, 0] + lm[151, 0]) / 2 * w)
+
+    textures = get_texture_map()[gender]
+    images, best_center = [], None
+    for order, (key, path) in enumerate(textures, start=1):
+        white = load_texture_rgba(path)
+        preview = render_hairline_overlay(image_bgr, ctx["points"], ext_faces, uv, white)
+        images.append({"hairline_type": key, "order": order, "image_bgr": preview})
+        if order == 1:  # 最佳发际线曲线的中点（面部中轴处的发际线 y）
+            overlay = build_overlay_layer(h, w, ctx["points"], ext_faces, uv, white)
+            ys, xs = np.where(overlay[:, :, 3] > 40)
+            if xs.size:
+                near = np.abs(xs - face_cx) <= max(2, int(w * 0.02))
+                col_ys = ys[near] if near.any() else ys[np.argsort(np.abs(xs - face_cx))[:20]]
+                best_center = (int(round(face_cx)), int(round(float(col_ys.mean()))))
+    return {"images": images, "best_center": best_center}
+
+
 def generate_grow_b(marked_bgr: np.ndarray, original_bgr: np.ndarray):
     """接口3：检测医生手绘发际线 → 遮罩 → 原图重画干净线 → ComfyUI 生发。