fix: SMTP配额重置使用北京时间

- 添加pytz时区支持
- 配额重置日期改为使用北京时间（UTC+8）
- 确保配额在北京时间凌晨0点重置，而不是UTC时间

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

email_service.py

@@ -19,6 +19,14 @@ import time
 import zipfile
 import secrets
 from datetime import datetime, timedelta
+import pytz
+
+# 北京时区
+BEIJING_TZ = pytz.timezone('Asia/Shanghai')
+
+def get_beijing_today():
+    """获取北京时间的今天日期字符串"""
+    return datetime.now(BEIJING_TZ).strftime('%Y-%m-%d')
 from email.mime.text import MIMEText
 from email.mime.multipart import MIMEMultipart
 from email.mime.base import MIMEBase
@@ -503,7 +511,7 @@ def _get_available_smtp_config(failover: bool = True) -> Optional[Dict[str, Any]
     获取可用的SMTP配置
     优先级: 主配置 > 按priority排序的启用配置
     """
-    today = datetime.now().strftime('%Y-%m-%d')
+    today = get_beijing_today()
 
     with db_pool.get_db() as conn:
         cursor = conn.cursor()
@@ -766,7 +774,7 @@ def send_email(
 
 def _get_next_available_smtp_config(exclude_ids: List[int]) -> Optional[Dict[str, Any]]:
     """获取下一个可用的SMTP配置（排除已尝试的）"""
-    today = datetime.now().strftime('%Y-%m-%d')
+    today = get_beijing_today()
 
     with db_pool.get_db() as conn:
         cursor = conn.cursor()