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/usagelog/usagelog.go

@@ -85,7 +85,7 @@ const (
 	UserColumn = "user_id"
 	// APIKeyTable is the table that holds the api_key relation/edge.
 	APIKeyTable = "usage_logs"
-	// APIKeyInverseTable is the table name for the ApiKey entity.
+	// APIKeyInverseTable is the table name for the APIKey entity.
 	// It exists in this package in order to avoid circular dependency with the "apikey" package.
 	APIKeyInverseTable = "api_keys"
 	// APIKeyColumn is the table column denoting the api_key relation/edge.

backend/ent/usagelog/where.go

@@ -1175,7 +1175,7 @@ func HasAPIKey() predicate.UsageLog {
 }
 
 // HasAPIKeyWith applies the HasEdge predicate on the "api_key" edge with a given conditions (other predicates).
-func HasAPIKeyWith(preds ...predicate.ApiKey) predicate.UsageLog {
+func HasAPIKeyWith(preds ...predicate.APIKey) predicate.UsageLog {
 	return predicate.UsageLog(func(s *sql.Selector) {
 		step := newAPIKeyStep()
 		sqlgraph.HasNeighborsWith(s, step, func(s *sql.Selector) {