fix(lint): 修复所有 Go 命名规范问题

- 全局替换 ApiKey → APIKey（类型、字段、方法、变量）
- 修复所有 initialism 命名（API, SMTP, HTML, URL 等）
- 添加所有缺失的包注释
- 修复导出符号的注释格式

主要修改：
- ApiKey → APIKey（所有出现的地方）
- ApiKeyID → APIKeyID
- ApiKeyIDs → APIKeyIDs
- TestSmtpConnection → TestSMTPConnection
- HtmlURL → HTMLURL
- 添加 20+ 个包注释
- 修复 10+ 个导出符号注释格式

验证结果：
- ✓ golangci-lint: 0 issues
- ✓ 单元测试: 通过
- ✓ 集成测试: 通过

backend/internal/service/email_service.go

@@ -40,8 +40,8 @@ const (
 	maxVerifyCodeAttempts = 5
 )
 
-// SmtpConfig SMTP配置
-type SmtpConfig struct {
+// SMTPConfig SMTP配置
+type SMTPConfig struct {
 	Host     string
 	Port     int
 	Username string
@@ -65,16 +65,16 @@ func NewEmailService(settingRepo SettingRepository, cache EmailCache) *EmailServ
 	}
 }
 
-// GetSmtpConfig 从数据库获取SMTP配置
-func (s *EmailService) GetSmtpConfig(ctx context.Context) (*SmtpConfig, error) {
+// GetSMTPConfig 从数据库获取SMTP配置
+func (s *EmailService) GetSMTPConfig(ctx context.Context) (*SMTPConfig, error) {
 	keys := []string{
-		SettingKeySmtpHost,
-		SettingKeySmtpPort,
-		SettingKeySmtpUsername,
-		SettingKeySmtpPassword,
-		SettingKeySmtpFrom,
-		SettingKeySmtpFromName,
-		SettingKeySmtpUseTLS,
+		SettingKeySMTPHost,
+		SettingKeySMTPPort,
+		SettingKeySMTPUsername,
+		SettingKeySMTPPassword,
+		SettingKeySMTPFrom,
+		SettingKeySMTPFromName,
+		SettingKeySMTPUseTLS,
 	}
 
 	settings, err := s.settingRepo.GetMultiple(ctx, keys)
@@ -82,34 +82,34 @@ func (s *EmailService) GetSmtpConfig(ctx context.Context) (*SmtpConfig, error) {
 		return nil, fmt.Errorf("get smtp settings: %w", err)
 	}
 
-	host := settings[SettingKeySmtpHost]
+	host := settings[SettingKeySMTPHost]
 	if host == "" {
 		return nil, ErrEmailNotConfigured
 	}
 
 	port := 587 // 默认端口
-	if portStr := settings[SettingKeySmtpPort]; portStr != "" {
+	if portStr := settings[SettingKeySMTPPort]; portStr != "" {
 		if p, err := strconv.Atoi(portStr); err == nil {
 			port = p
 		}
 	}
 
-	useTLS := settings[SettingKeySmtpUseTLS] == "true"
+	useTLS := settings[SettingKeySMTPUseTLS] == "true"
 
-	return &SmtpConfig{
+	return &SMTPConfig{
 		Host:     host,
 		Port:     port,
-		Username: settings[SettingKeySmtpUsername],
-		Password: settings[SettingKeySmtpPassword],
-		From:     settings[SettingKeySmtpFrom],
-		FromName: settings[SettingKeySmtpFromName],
+		Username: settings[SettingKeySMTPUsername],
+		Password: settings[SettingKeySMTPPassword],
+		From:     settings[SettingKeySMTPFrom],
+		FromName: settings[SettingKeySMTPFromName],
 		UseTLS:   useTLS,
 	}, nil
 }
 
 // SendEmail 发送邮件（使用数据库中保存的配置）
 func (s *EmailService) SendEmail(ctx context.Context, to, subject, body string) error {
-	config, err := s.GetSmtpConfig(ctx)
+	config, err := s.GetSMTPConfig(ctx)
 	if err != nil {
 		return err
 	}
@@ -117,7 +117,7 @@ func (s *EmailService) SendEmail(ctx context.Context, to, subject, body string)
 }
 
 // SendEmailWithConfig 使用指定配置发送邮件
-func (s *EmailService) SendEmailWithConfig(config *SmtpConfig, to, subject, body string) error {
+func (s *EmailService) SendEmailWithConfig(config *SMTPConfig, to, subject, body string) error {
 	from := config.From
 	if config.FromName != "" {
 		from = fmt.Sprintf("%s <%s>", config.FromName, config.From)
@@ -306,8 +306,8 @@ func (s *EmailService) buildVerifyCodeEmailBody(code, siteName string) string {
 `, siteName, code)
 }
 
-// TestSmtpConnectionWithConfig 使用指定配置测试SMTP连接
-func (s *EmailService) TestSmtpConnectionWithConfig(config *SmtpConfig) error {
+// TestSMTPConnectionWithConfig 使用指定配置测试SMTP连接
+func (s *EmailService) TestSMTPConnectionWithConfig(config *SMTPConfig) error {
 	addr := fmt.Sprintf("%s:%d", config.Host, config.Port)
 
 	if config.UseTLS {