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/handler/dto/mappers.go

@@ -1,3 +1,4 @@
+// Package dto provides data transfer objects for HTTP handlers.
 package dto
 
 import "github.com/Wei-Shaw/sub2api/internal/service"
@@ -26,11 +27,11 @@ func UserFromService(u *service.User) *User {
 		return nil
 	}
 	out := UserFromServiceShallow(u)
-	if len(u.ApiKeys) > 0 {
-		out.ApiKeys = make([]ApiKey, 0, len(u.ApiKeys))
-		for i := range u.ApiKeys {
-			k := u.ApiKeys[i]
-			out.ApiKeys = append(out.ApiKeys, *ApiKeyFromService(&k))
+	if len(u.APIKeys) > 0 {
+		out.APIKeys = make([]APIKey, 0, len(u.APIKeys))
+		for i := range u.APIKeys {
+			k := u.APIKeys[i]
+			out.APIKeys = append(out.APIKeys, *APIKeyFromService(&k))
 		}
 	}
 	if len(u.Subscriptions) > 0 {
@@ -43,11 +44,11 @@ func UserFromService(u *service.User) *User {
 	return out
 }
 
-func ApiKeyFromService(k *service.ApiKey) *ApiKey {
+func APIKeyFromService(k *service.APIKey) *APIKey {
 	if k == nil {
 		return nil
 	}
-	return &ApiKey{
+	return &APIKey{
 		ID:        k.ID,
 		UserID:    k.UserID,
 		Key:       k.Key,
@@ -222,7 +223,7 @@ func UsageLogFromService(l *service.UsageLog) *UsageLog {
 	return &UsageLog{
 		ID:                    l.ID,
 		UserID:                l.UserID,
-		ApiKeyID:              l.ApiKeyID,
+		APIKeyID:              l.APIKeyID,
 		AccountID:             l.AccountID,
 		RequestID:             l.RequestID,
 		Model:                 l.Model,
@@ -247,7 +248,7 @@ func UsageLogFromService(l *service.UsageLog) *UsageLog {
 		FirstTokenMs:          l.FirstTokenMs,
 		CreatedAt:             l.CreatedAt,
 		User:                  UserFromServiceShallow(l.User),
-		ApiKey:                ApiKeyFromService(l.ApiKey),
+		APIKey:                APIKeyFromService(l.APIKey),
 		Account:               AccountFromService(l.Account),
 		Group:                 GroupFromServiceShallow(l.Group),
 		Subscription:          UserSubscriptionFromService(l.Subscription),

backend/internal/handler/dto/types.go

@@ -15,11 +15,11 @@ type User struct {
 	CreatedAt     time.Time `json:"created_at"`
 	UpdatedAt     time.Time `json:"updated_at"`
 
-	ApiKeys       []ApiKey           `json:"api_keys,omitempty"`
+	APIKeys       []APIKey           `json:"api_keys,omitempty"`
 	Subscriptions []UserSubscription `json:"subscriptions,omitempty"`
 }
 
-type ApiKey struct {
+type APIKey struct {
 	ID        int64     `json:"id"`
 	UserID    int64     `json:"user_id"`
 	Key       string    `json:"key"`
@@ -139,7 +139,7 @@ type RedeemCode struct {
 type UsageLog struct {
 	ID        int64  `json:"id"`
 	UserID    int64  `json:"user_id"`
-	ApiKeyID  int64  `json:"api_key_id"`
+	APIKeyID  int64  `json:"api_key_id"`
 	AccountID int64  `json:"account_id"`
 	RequestID string `json:"request_id"`
 	Model     string `json:"model"`
@@ -171,7 +171,7 @@ type UsageLog struct {
 	CreatedAt time.Time `json:"created_at"`
 
 	User         *User             `json:"user,omitempty"`
-	ApiKey       *ApiKey           `json:"api_key,omitempty"`
+	APIKey       *APIKey           `json:"api_key,omitempty"`
 	Account      *Account          `json:"account,omitempty"`
 	Group        *Group            `json:"group,omitempty"`
 	Subscription *UserSubscription `json:"subscription,omitempty"`