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/hook/hook.go

@@ -9,6 +9,18 @@ import (
 	"github.com/Wei-Shaw/sub2api/ent"
 )
 
+// The APIKeyFunc type is an adapter to allow the use of ordinary
+// function as APIKey mutator.
+type APIKeyFunc func(context.Context, *ent.APIKeyMutation) (ent.Value, error)
+
+// Mutate calls f(ctx, m).
+func (f APIKeyFunc) Mutate(ctx context.Context, m ent.Mutation) (ent.Value, error) {
+	if mv, ok := m.(*ent.APIKeyMutation); ok {
+		return f(ctx, mv)
+	}
+	return nil, fmt.Errorf("unexpected mutation type %T. expect *ent.APIKeyMutation", m)
+}
+
 // The AccountFunc type is an adapter to allow the use of ordinary
 // function as Account mutator.
 type AccountFunc func(context.Context, *ent.AccountMutation) (ent.Value, error)
@@ -33,18 +45,6 @@ func (f AccountGroupFunc) Mutate(ctx context.Context, m ent.Mutation) (ent.Value
 	return nil, fmt.Errorf("unexpected mutation type %T. expect *ent.AccountGroupMutation", m)
 }
 
-// The ApiKeyFunc type is an adapter to allow the use of ordinary
-// function as ApiKey mutator.
-type ApiKeyFunc func(context.Context, *ent.ApiKeyMutation) (ent.Value, error)
-
-// Mutate calls f(ctx, m).
-func (f ApiKeyFunc) Mutate(ctx context.Context, m ent.Mutation) (ent.Value, error) {
-	if mv, ok := m.(*ent.ApiKeyMutation); ok {
-		return f(ctx, mv)
-	}
-	return nil, fmt.Errorf("unexpected mutation type %T. expect *ent.ApiKeyMutation", m)
-}
-
 // The GroupFunc type is an adapter to allow the use of ordinary
 // function as Group mutator.
 type GroupFunc func(context.Context, *ent.GroupMutation) (ent.Value, error)