Add email sending toggle and fix event timestamp default

Introduces enable_email_sending to control real email delivery in config. Updates Event.created_at to use default_factory for correct timestamp initialization.

app/config.py

@@ -119,6 +119,7 @@ class Settings(BaseSettings):
 
     # 邮件服务设置
     enable_email_verification: bool = Field(default=False, description="是否启用邮件验证功能")
+    enable_email_sending: bool = Field(default=False, description="是否真实发送邮件（false时仅模拟发送，输出到日志）")
     smtp_server: str = "localhost"
     smtp_port: int = 587
     smtp_username: str = ""