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/user.go

@@ -57,7 +57,7 @@ const (
 	Table = "users"
 	// APIKeysTable is the table that holds the api_keys relation/edge.
 	APIKeysTable = "api_keys"
-	// APIKeysInverseTable is the table name for the ApiKey entity.
+	// APIKeysInverseTable is the table name for the APIKey entity.
 	// It exists in this package in order to avoid circular dependency with the "apikey" package.
 	APIKeysInverseTable = "api_keys"
 	// APIKeysColumn is the table column denoting the api_keys relation/edge.

backend/ent/user/where.go

@@ -722,7 +722,7 @@ func HasAPIKeys() predicate.User {
 }
 
 // HasAPIKeysWith applies the HasEdge predicate on the "api_keys" edge with a given conditions (other predicates).
-func HasAPIKeysWith(preds ...predicate.ApiKey) predicate.User {
+func HasAPIKeysWith(preds ...predicate.APIKey) predicate.User {
 	return predicate.User(func(s *sql.Selector) {
 		step := newAPIKeysStep()
 		sqlgraph.HasNeighborsWith(s, step, func(s *sql.Selector) {