feat(接口2): 新增生发后图片(ComfyUI/Flux inpaint)

在发际线预览基础上，每种发际线再出一张「植发3个月」生发图：

- hairline/mask.py: headmark 5步法遮罩(额头上部区域∩SegFormer头部=ROI，
  取发际线曲线以上闭合区域)；用 hairline_texture_black 渲染黑线替代手绘检测；
  compose_comfy_rgba 合成 RGBA(alpha=255-mask, 透明=重绘区, 对齐 ComfyUI mask=1-alpha)
- hairline/comfyui.py: ComfyUI 客户端(默认8182)，/upload/image+/prompt(改节点26+随机seed)
  +轮询/history+/view 取回生发图
- hairline/render.py: 抽出 build_overlay_layer 供遮罩取曲线像素
- hairline/service.py: extract_context 一次出 landmarks/parse_map/502点；
  generate_grow_results 每种=预览+生发图(同步串行N张，单张ComfyUI失败则grown置空不拖垮整请求)
- app.py: /hair/grow 返回 results[].grown_image_base64；重活放线程池避免卡事件循环
- add_hair.json 工作流 + hairline_texture_black/ 黑贴图入库
- 测试: test_mask.py(遮罩几何) + test_api mock ComfyUI 验 grown 字段，35 全绿

实测(5090): female 5张生发图同步约18s；预览/生发图人物五官服饰背景保持、黑线已清除。

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

app.py

@@ -504,17 +504,22 @@ async def hair_grow(
         return err(1002, "人像分辨率过低")
 
     try:
-        from hairline.service import generate_previews
+        from fastapi.concurrency import run_in_threadpool
+        from hairline.service import generate_grow_results
 
-        previews = generate_previews(image, gender)   # 无人脸 → None
-        if previews is None:
+        # 预览 + 生发(ComfyUI) 都是阻塞且较慢，放线程池避免卡住事件循环
+        items = await run_in_threadpool(generate_grow_results, image, gender)
+        if items is None:
             return err(1001, "无法识别人像")
 
         results = []
-        for p in previews:
-            ok_enc, png = cv2.imencode(".png", p["image_bgr"])
+        for p in items:
+            _ok, png = cv2.imencode(".png", p["image_bgr"])
+            grown_b64 = (base64.b64encode(p["grown_png"]).decode()
+                         if p["grown_png"] else None)
             results.append({
                 "image_base64": base64.b64encode(png.tobytes()).decode(),
+                "grown_image_base64": grown_b64,
                 "hairline_type": p["hairline_type"],
                 "order": p["order"],
             })