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_create.go

@@ -166,14 +166,14 @@ func (_c *UserCreate) SetNillableNotes(v *string) *UserCreate {
 	return _c
 }
 
-// AddAPIKeyIDs adds the "api_keys" edge to the ApiKey entity by IDs.
+// AddAPIKeyIDs adds the "api_keys" edge to the APIKey entity by IDs.
 func (_c *UserCreate) AddAPIKeyIDs(ids ...int64) *UserCreate {
 	_c.mutation.AddAPIKeyIDs(ids...)
 	return _c
 }
 
-// AddAPIKeys adds the "api_keys" edges to the ApiKey entity.
-func (_c *UserCreate) AddAPIKeys(v ...*ApiKey) *UserCreate {
+// AddAPIKeys adds the "api_keys" edges to the APIKey entity.
+func (_c *UserCreate) AddAPIKeys(v ...*APIKey) *UserCreate {
 	ids := make([]int64, len(v))
 	for i := range v {
 		ids[i] = v[i].ID