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/ent/tx.go

@@ -14,12 +14,12 @@ import (
 // Tx is a transactional client that is created by calling Client.Tx().
 type Tx struct {
 	config
+	// APIKey is the client for interacting with the APIKey builders.
+	APIKey *APIKeyClient
 	// Account is the client for interacting with the Account builders.
 	Account *AccountClient
 	// AccountGroup is the client for interacting with the AccountGroup builders.
 	AccountGroup *AccountGroupClient
-	// ApiKey is the client for interacting with the ApiKey builders.
-	ApiKey *ApiKeyClient
 	// Group is the client for interacting with the Group builders.
 	Group *GroupClient
 	// Proxy is the client for interacting with the Proxy builders.
@@ -171,9 +171,9 @@ func (tx *Tx) Client() *Client {
 }
 
 func (tx *Tx) init() {
+	tx.APIKey = NewAPIKeyClient(tx.config)
 	tx.Account = NewAccountClient(tx.config)
 	tx.AccountGroup = NewAccountGroupClient(tx.config)
-	tx.ApiKey = NewApiKeyClient(tx.config)
 	tx.Group = NewGroupClient(tx.config)
 	tx.Proxy = NewProxyClient(tx.config)
 	tx.RedeemCode = NewRedeemCodeClient(tx.config)
@@ -193,7 +193,7 @@ func (tx *Tx) init() {
 // of them in order to commit or rollback the transaction.
 //
 // If a closed transaction is embedded in one of the generated entities, and the entity
-// applies a query, for example: Account.QueryXXX(), the query will be executed
+// applies a query, for example: APIKey.QueryXXX(), the query will be executed
 // through the driver which created this transaction.
 //
 // Note that txDriver is not goroutine safe.