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/routes/gateway.go

@@ -13,8 +13,8 @@ import (
 func RegisterGatewayRoutes(
 	r *gin.Engine,
 	h *handler.Handlers,
-	apiKeyAuth middleware.ApiKeyAuthMiddleware,
-	apiKeyService *service.ApiKeyService,
+	apiKeyAuth middleware.APIKeyAuthMiddleware,
+	apiKeyService *service.APIKeyService,
 	subscriptionService *service.SubscriptionService,
 	cfg *config.Config,
 ) {
@@ -36,7 +36,7 @@ func RegisterGatewayRoutes(
 	// Gemini 原生 API 兼容层（Gemini SDK/CLI 直连）
 	gemini := r.Group("/v1beta")
 	gemini.Use(bodyLimit)
-	gemini.Use(middleware.ApiKeyAuthWithSubscriptionGoogle(apiKeyService, subscriptionService, cfg))
+	gemini.Use(middleware.APIKeyAuthWithSubscriptionGoogle(apiKeyService, subscriptionService, cfg))
 	{
 		gemini.GET("/models", h.Gateway.GeminiV1BetaListModels)
 		gemini.GET("/models/:model", h.Gateway.GeminiV1BetaGetModel)
@@ -65,7 +65,7 @@ func RegisterGatewayRoutes(
 	antigravityV1Beta := r.Group("/antigravity/v1beta")
 	antigravityV1Beta.Use(bodyLimit)
 	antigravityV1Beta.Use(middleware.ForcePlatform(service.PlatformAntigravity))
-	antigravityV1Beta.Use(middleware.ApiKeyAuthWithSubscriptionGoogle(apiKeyService, subscriptionService, cfg))
+	antigravityV1Beta.Use(middleware.APIKeyAuthWithSubscriptionGoogle(apiKeyService, subscriptionService, cfg))
 	{
 		antigravityV1Beta.GET("/models", h.Gateway.GeminiV1BetaListModels)
 		antigravityV1Beta.GET("/models/:model", h.Gateway.GeminiV1BetaGetModel)