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/usagelog.go

@@ -83,7 +83,7 @@ type UsageLogEdges struct {
 	// User holds the value of the user edge.
 	User *User `json:"user,omitempty"`
 	// APIKey holds the value of the api_key edge.
-	APIKey *ApiKey `json:"api_key,omitempty"`
+	APIKey *APIKey `json:"api_key,omitempty"`
 	// Account holds the value of the account edge.
 	Account *Account `json:"account,omitempty"`
 	// Group holds the value of the group edge.
@@ -108,7 +108,7 @@ func (e UsageLogEdges) UserOrErr() (*User, error) {
 
 // APIKeyOrErr returns the APIKey value or an error if the edge
 // was not loaded in eager-loading, or loaded but was not found.
-func (e UsageLogEdges) APIKeyOrErr() (*ApiKey, error) {
+func (e UsageLogEdges) APIKeyOrErr() (*APIKey, error) {
 	if e.APIKey != nil {
 		return e.APIKey, nil
 	} else if e.loadedTypes[1] {
@@ -359,7 +359,7 @@ func (_m *UsageLog) QueryUser() *UserQuery {
 }
 
 // QueryAPIKey queries the "api_key" edge of the UsageLog entity.
-func (_m *UsageLog) QueryAPIKey() *ApiKeyQuery {
+func (_m *UsageLog) QueryAPIKey() *APIKeyQuery {
 	return NewUsageLogClient(_m.config).QueryAPIKey(_m)
 }