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/schema/api_key.go

@@ -12,25 +12,25 @@ import (
 	"entgo.io/ent/schema/index"
 )
 
-// ApiKey holds the schema definition for the ApiKey entity.
-type ApiKey struct {
+// APIKey holds the schema definition for the APIKey entity.
+type APIKey struct {
 	ent.Schema
 }
 
-func (ApiKey) Annotations() []schema.Annotation {
+func (APIKey) Annotations() []schema.Annotation {
 	return []schema.Annotation{
 		entsql.Annotation{Table: "api_keys"},
 	}
 }
 
-func (ApiKey) Mixin() []ent.Mixin {
+func (APIKey) Mixin() []ent.Mixin {
 	return []ent.Mixin{
 		mixins.TimeMixin{},
 		mixins.SoftDeleteMixin{},
 	}
 }
 
-func (ApiKey) Fields() []ent.Field {
+func (APIKey) Fields() []ent.Field {
 	return []ent.Field{
 		field.Int64("user_id"),
 		field.String("key").
@@ -49,7 +49,7 @@ func (ApiKey) Fields() []ent.Field {
 	}
 }
 
-func (ApiKey) Edges() []ent.Edge {
+func (APIKey) Edges() []ent.Edge {
 	return []ent.Edge{
 		edge.From("user", User.Type).
 			Ref("api_keys").
@@ -64,7 +64,7 @@ func (ApiKey) Edges() []ent.Edge {
 	}
 }
 
-func (ApiKey) Indexes() []ent.Index {
+func (APIKey) Indexes() []ent.Index {
 	return []ent.Index{
 		// key 字段已在 Fields() 中声明 Unique()，无需重复索引
 		index.Fields("user_id"),

backend/ent/schema/group.go

@@ -77,7 +77,7 @@ func (Group) Fields() []ent.Field {
 
 func (Group) Edges() []ent.Edge {
 	return []ent.Edge{
-		edge.To("api_keys", ApiKey.Type),
+		edge.To("api_keys", APIKey.Type),
 		edge.To("redeem_codes", RedeemCode.Type),
 		edge.To("subscriptions", UserSubscription.Type),
 		edge.To("usage_logs", UsageLog.Type),

backend/ent/schema/usage_log.go

@@ -113,7 +113,7 @@ func (UsageLog) Edges() []ent.Edge {
 			Field("user_id").
 			Required().
 			Unique(),
-		edge.From("api_key", ApiKey.Type).
+		edge.From("api_key", APIKey.Type).
 			Ref("usage_logs").
 			Field("api_key_id").
 			Required().

backend/ent/schema/user.go

@@ -66,7 +66,7 @@ func (User) Fields() []ent.Field {
 
 func (User) Edges() []ent.Edge {
 	return []ent.Edge{
-		edge.To("api_keys", ApiKey.Type),
+		edge.To("api_keys", APIKey.Type),
 		edge.To("redeem_codes", RedeemCode.Type),
 		edge.To("subscriptions", UserSubscription.Type),
 		edge.To("assigned_subscriptions", UserSubscription.Type),