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/user.go

@@ -48,7 +48,7 @@ type User struct {
 // UserEdges holds the relations/edges for other nodes in the graph.
 type UserEdges struct {
 	// APIKeys holds the value of the api_keys edge.
-	APIKeys []*ApiKey `json:"api_keys,omitempty"`
+	APIKeys []*APIKey `json:"api_keys,omitempty"`
 	// RedeemCodes holds the value of the redeem_codes edge.
 	RedeemCodes []*RedeemCode `json:"redeem_codes,omitempty"`
 	// Subscriptions holds the value of the subscriptions edge.
@@ -70,7 +70,7 @@ type UserEdges struct {
 
 // APIKeysOrErr returns the APIKeys value or an error if the edge
 // was not loaded in eager-loading.
-func (e UserEdges) APIKeysOrErr() ([]*ApiKey, error) {
+func (e UserEdges) APIKeysOrErr() ([]*APIKey, error) {
 	if e.loadedTypes[0] {
 		return e.APIKeys, nil
 	}
@@ -255,7 +255,7 @@ func (_m *User) Value(name string) (ent.Value, error) {
 }
 
 // QueryAPIKeys queries the "api_keys" edge of the User entity.
-func (_m *User) QueryAPIKeys() *ApiKeyQuery {
+func (_m *User) QueryAPIKeys() *APIKeyQuery {
 	return NewUserClient(_m.config).QueryAPIKeys(_m)
 }