add default avatar gen is google avatar not exists

app/auth.py

@@ -86,11 +86,18 @@ async def auth_callback(request: Request, code: str, state: str):
             random_suffix = secrets.token_hex(3) # 生成 6 位随机字符
             suggested_handle = f"{base_handle}_{random_suffix}"
                         
+            # 如果 Google 没有头像，使用 DiceBear 头像生成器作为兜底
+            google_avatar = user_info.get("picture")
+            if not google_avatar:
+                default_avatar_url = f"https://api.dicebear.com/7.x/lorelei/svg?seed={base_handle}"
+            else:
+                default_avatar_url = google_avatar
+            
             user = User(
                 google_id=user_info["id"],
                 email=user_info["email"],
                 name=user_info["name"],
-                avatar=user_info["picture"],
+                avatar=default_avatar_url,
                 handle=suggested_handle,
             )
             db.add(user)