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>

tests/test_api.py

@@ -82,9 +82,10 @@ def test_grow_missing_gender_1004(client):
     assert r.json()["code"] == 1004
 
 
-_PNG_1x1 = base64.b64decode(
-    "iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAQAAAC1HAwCAAAAC0lEQVR42mNk+M9QDwADhgGAWjR9awAAAABJRU5ErkJggg=="
-)
+# mock ComfyUI 输出：一张合法 PNG（worker 会把它重编码成 JPG）
+import cv2 as _cv2
+import numpy as _np
+_PNG_1x1 = _cv2.imencode(".png", _np.full((8, 8, 3), 200, _np.uint8))[1].tobytes()
 
 
 def test_grow_female_returns_5(client, monkeypatch):
@@ -99,8 +100,8 @@ def test_grow_female_returns_5(client, monkeypatch):
     results = body["data"]["results"]
     assert [x["hairline_type"] for x in results] == ["ellipse", "flower", "heart", "straight", "wave"]
     assert [x["order"] for x in results] == [1, 2, 3, 4, 5]
-    assert base64.b64decode(results[0]["image_base64"])[:8] == b"\x89PNG\r\n\x1a\n"
-    assert base64.b64decode(results[0]["grown_image_base64"])[:8] == b"\x89PNG\r\n\x1a\n"
+    assert base64.b64decode(results[0]["image_base64"])[:3] == b"\xff\xd8\xff"        # JPEG
+    assert base64.b64decode(results[0]["grown_image_base64"])[:3] == b"\xff\xd8\xff"  # JPEG
     assert "image_url" not in results[0]
 
 
@@ -126,7 +127,7 @@ def test_growb_success(client, monkeypatch):
     assert body["code"] == 0, body
     d = body["data"]
     assert d["hairline_type"] == "custom"
-    assert base64.b64decode(d["hair_growth_image_base64"])[:8] == b"\x89PNG\r\n\x1a\n"
+    assert base64.b64decode(d["hair_growth_image_base64"])[:3] == b"\xff\xd8\xff"     # JPEG
     assert "best_hairline_image_base64" not in d            # 已去掉该字段
     assert "best_hairline_image_url" not in d
 
@@ -145,7 +146,7 @@ def test_hairline_gen_female(client):
     assert body["code"] == 0, body
     d = body["data"]
     assert [x["order"] for x in d["hairline_images"]] == [1, 2, 3, 4, 5]
-    assert base64.b64decode(d["hairline_images"][0]["image_base64"])[:8] == b"\x89PNG\r\n\x1a\n"
+    assert base64.b64decode(d["hairline_images"][0]["image_base64"])[:3] == b"\xff\xd8\xff"  # JPEG
     c = d["best_hairline_center_point"]
     assert 0 <= c["x"] <= 682 and 0 <= c["y"] <= 811        # 落在原图范围内
     assert "image_url" not in d["hairline_images"][0]