fix(auth): 添加验证码生成异常处理

捕获验证码生成过程中的异常并抛出HTTP异常，提高系统健壮性

backend/auth/utils/captcha.py

@@ -5,6 +5,7 @@ from utils.redis_client import get_redis, set_token_in_redis
 import redis.asyncio as redis
 import random
 import string
+from fastapi import HTTPException
 
 
 def generate_captcha(length: int = 4) -> tuple[str, bytes]:
@@ -13,6 +14,7 @@ def generate_captcha(length: int = 4) -> tuple[str, bytes]:
     characters = string.digits + string.ascii_uppercase
     captcha_text = ''.join(random.choices(characters, k=length))
     
+    try:
         # 生成验证码图片
         image = ImageCaptcha(width=150, height=50)
         image_bytes = image.generate(captcha_text)
@@ -23,6 +25,8 @@ def generate_captcha(length: int = 4) -> tuple[str, bytes]:
         buffer.seek(0)
         
         return captcha_text, buffer.getvalue()
+    except Exception as e:
+        raise HTTPException(status_code=500, detail="验证码生成失败")
 
 
 async def save_captcha_to_redis(redis_conn: redis.Redis, captcha_id: str, captcha_text: str, expire_seconds: int = 300):