feat(email): 添加发件人邮箱字段并优化邮件发送逻辑

添加独立的sender_email字段作为发件人邮箱，优先使用该字段而非smtp_username
更新相关表单、模型和测试用例以支持新字段
重构邮件发送逻辑，统一邮箱格式验证和错误提示

core/forms.py

@@ -62,12 +62,13 @@ class SystemConfigForm(forms.ModelForm):
     """系统配置表单"""
     class Meta:
         model = SystemConfig
-        fields = ['smtp_server', 'smtp_port', 'smtp_username', 'smtp_password', 'send_time', 'recipient_email']
+        fields = ['smtp_server', 'smtp_port', 'smtp_username', 'smtp_password', 'sender_email', 'send_time', 'recipient_email']
         widgets = {
             'smtp_server': forms.TextInput(attrs={'class': 'form-control', 'placeholder': '请输入SMTP服务器'}),
             'smtp_port': forms.NumberInput(attrs={'class': 'form-control', 'placeholder': '请输入SMTP端口'}),
             'smtp_username': forms.TextInput(attrs={'class': 'form-control', 'placeholder': '请输入SMTP用户名'}),
             'smtp_password': forms.PasswordInput(attrs={'class': 'form-control', 'placeholder': '请输入SMTP密码'}),
+            'sender_email': forms.EmailInput(attrs={'class': 'form-control', 'placeholder': '请输入发件人邮箱'}),
             'send_time': forms.TimeInput(attrs={'type': 'time', 'class': 'form-control'}),
             'recipient_email': forms.EmailInput(attrs={'class': 'form-control', 'placeholder': '请输入收件人邮箱'}),
         }