feat(server): 接入 QQ 邮箱 SMTP 发送登录验证码

- 支持 465 隐式 TLS 与 587 STARTTLS，SMTP 配置优先、Postmark 备用
- 发件人支持 QQ 域名邮箱地址（z@kepo.net），凭证仅走环境变量
- HTTP 测试覆盖发送成功与认证失败；真实发信测试需显式开启 RUN_QQ_SMTP_LIVE_TEST

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

server/internal/config/config.go

@@ -14,6 +14,7 @@ type Config struct {
 	Stripe     StripeConfig
 	BinancePay BinancePayConfig
 	Postmark   PostmarkConfig
+	SMTP       SMTPConfig
 	Commerce   CommerceConfig
 	Admin      AdminConfig
 	Wiki       WikiConfig
@@ -71,6 +72,20 @@ type PostmarkConfig struct {
 	APIBaseURL  string
 }
 
+type SMTPConfig struct {
+	Host      string
+	Port      string
+	Username  string
+	Password  string
+	FromEmail string
+	TLSMode   string
+	TLSCAFile string
+}
+
+func (c SMTPConfig) IsConfigured() bool {
+	return c.Host != "" || c.Username != "" || c.Password != "" || c.FromEmail != ""
+}
+
 type CommerceConfig struct {
 	PublicBaseURL string
 	WebAppURL     string
@@ -136,6 +151,15 @@ func Load() *Config {
 			FromEmail:   getEnv("POSTMARK_FROM_EMAIL", ""),
 			APIBaseURL:  getEnv("POSTMARK_API_BASE_URL", "https://api.postmarkapp.com"),
 		},
+		SMTP: SMTPConfig{
+			Host:      getEnv("SMTP_HOST", ""),
+			Port:      getEnv("SMTP_PORT", "465"),
+			Username:  getEnv("SMTP_USERNAME", ""),
+			Password:  getEnv("SMTP_PASSWORD", ""),
+			FromEmail: getEnv("SMTP_FROM_EMAIL", ""),
+			TLSMode:   getEnv("SMTP_TLS_MODE", "implicit"),
+			TLSCAFile: getEnv("SMTP_TLS_CA_FILE", ""),
+		},
 		Commerce: CommerceConfig{
 			PublicBaseURL: getEnv("PUBLIC_BASE_URL", "http://localhost:8080"),
 			WebAppURL:     getEnv("WEB_APP_URL", "http://localhost:4260"),