perf(图片): 接口2/3/5 返回 JPG(体积~9×↓)，接口1 标注图仍 PNG(透明)

- app.py: 接口2(预览+生发)/3(生发)/5(发际线叠图) 编码改 JPG(质量90,env JPG_QUALITY);
  接口1 annotated_image 含透明仍 PNG。_png_to_jpg_b64 把 ComfyUI 的 PNG 重编码为 JPG(无法解码则透传)
- gateway/forward.py: 落盘按内容嗅探扩展名(PNG头→.png 否则.jpg)，原先硬编码 .png
- 测试/文档同步；实测接口5 一张 59KB(JPG) vs 548KB(PNG)。pytest 44 全绿

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

app.py

@@ -226,6 +226,24 @@ async def resolve_image_bytes(image_file, image_url, image_base64):
         return None, err(1008, "base64 解码失败")
 
 
+# 接口2/3/5 返回不透明照片，用 JPG 显著减小体积（接口1 标注图含透明，仍用 PNG）
+_JPG_QUALITY = int(os.getenv("JPG_QUALITY", "90"))
+
+
+def _jpg_b64(bgr) -> str:
+    """BGR 图 → JPG base64。"""
+    _ok, buf = cv2.imencode(".jpg", bgr, [cv2.IMWRITE_JPEG_QUALITY, _JPG_QUALITY])
+    return base64.b64encode(buf.tobytes()).decode()
+
+
+def _png_to_jpg_b64(png_bytes) -> str:
+    """ComfyUI 返回的 PNG 字节 → 重编码为 JPG base64；无法解码则原样透传。"""
+    img = cv2.imdecode(np.frombuffer(png_bytes, np.uint8), cv2.IMREAD_COLOR)
+    if img is None:
+        return base64.b64encode(png_bytes).decode()
+    return _jpg_b64(img)
+
+
 # ---------------------------------------------------------------------------
 # 通用图片请求 Body（JSON 方式，用于 url / base64）
 # ---------------------------------------------------------------------------
@@ -514,12 +532,10 @@ async def hair_grow(
 
         results = []
         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,
+                "image_base64": _jpg_b64(p["image_bgr"]),               # 预览图 JPG
+                "grown_image_base64": (_png_to_jpg_b64(p["grown_png"])  # 生发图 JPG
+                                       if p["grown_png"] else None),
                 "hairline_type": p["hairline_type"],
                 "order": p["order"],
             })
@@ -602,7 +618,7 @@ async def hair_grow_b(
         if res["status"] == "no_line":
             return err(1001, "未检测到发际线划线，请确认划线图额头有清晰的手绘发际线")
 
-        grown_b64 = base64.b64encode(res["grown_png"]).decode() if res["grown_png"] else None
+        grown_b64 = _png_to_jpg_b64(res["grown_png"]) if res["grown_png"] else None  # 生发图 JPG
         data = {
             "hair_growth_image_base64": grown_b64,
             "hairline_type": "custom",
@@ -783,9 +799,8 @@ async def hairline_generate(
 
         hairline_images = []
         for it in res["images"]:
-            _ok, png = cv2.imencode(".png", it["image_bgr"])
             hairline_images.append({
-                "image_base64": base64.b64encode(png.tobytes()).decode(),
+                "image_base64": _jpg_b64(it["image_bgr"]),     # 发际线叠图 JPG
                 "order": it["order"],
             })
         c = res["best_center"]