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/internal/server/router.go

@@ -17,8 +17,8 @@ func SetupRouter(
 	handlers *handler.Handlers,
 	jwtAuth middleware2.JWTAuthMiddleware,
 	adminAuth middleware2.AdminAuthMiddleware,
-	apiKeyAuth middleware2.ApiKeyAuthMiddleware,
-	apiKeyService *service.ApiKeyService,
+	apiKeyAuth middleware2.APIKeyAuthMiddleware,
+	apiKeyService *service.APIKeyService,
 	subscriptionService *service.SubscriptionService,
 	cfg *config.Config,
 ) *gin.Engine {
@@ -43,8 +43,8 @@ func registerRoutes(
 	h *handler.Handlers,
 	jwtAuth middleware2.JWTAuthMiddleware,
 	adminAuth middleware2.AdminAuthMiddleware,
-	apiKeyAuth middleware2.ApiKeyAuthMiddleware,
-	apiKeyService *service.ApiKeyService,
+	apiKeyAuth middleware2.APIKeyAuthMiddleware,
+	apiKeyService *service.APIKeyService,
 	subscriptionService *service.SubscriptionService,
 	cfg *config.Config,
 ) {