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/predicate/predicate.go

@@ -6,15 +6,15 @@ import (
 	"entgo.io/ent/dialect/sql"
 )
 
+// APIKey is the predicate function for apikey builders.
+type APIKey func(*sql.Selector)
+
 // Account is the predicate function for account builders.
 type Account func(*sql.Selector)
 
 // AccountGroup is the predicate function for accountgroup builders.
 type AccountGroup func(*sql.Selector)
 
-// ApiKey is the predicate function for apikey builders.
-type ApiKey func(*sql.Selector)
-
 // Group is the predicate function for group builders.
 type Group func(*sql.Selector)