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/handler/gemini_v1beta_handler.go

@@ -21,7 +21,7 @@ import (
 // GeminiV1BetaListModels proxies:
 // GET /v1beta/models
 func (h *GatewayHandler) GeminiV1BetaListModels(c *gin.Context) {
-	apiKey, ok := middleware.GetApiKeyFromContext(c)
+	apiKey, ok := middleware.GetAPIKeyFromContext(c)
 	if !ok || apiKey == nil {
 		googleError(c, http.StatusUnauthorized, "Invalid API key")
 		return
@@ -67,7 +67,7 @@ func (h *GatewayHandler) GeminiV1BetaListModels(c *gin.Context) {
 // GeminiV1BetaGetModel proxies:
 // GET /v1beta/models/{model}
 func (h *GatewayHandler) GeminiV1BetaGetModel(c *gin.Context) {
-	apiKey, ok := middleware.GetApiKeyFromContext(c)
+	apiKey, ok := middleware.GetAPIKeyFromContext(c)
 	if !ok || apiKey == nil {
 		googleError(c, http.StatusUnauthorized, "Invalid API key")
 		return
@@ -120,7 +120,7 @@ func (h *GatewayHandler) GeminiV1BetaGetModel(c *gin.Context) {
 // POST /v1beta/models/{model}:generateContent
 // POST /v1beta/models/{model}:streamGenerateContent?alt=sse
 func (h *GatewayHandler) GeminiV1BetaModels(c *gin.Context) {
-	apiKey, ok := middleware.GetApiKeyFromContext(c)
+	apiKey, ok := middleware.GetAPIKeyFromContext(c)
 	if !ok || apiKey == nil {
 		googleError(c, http.StatusUnauthorized, "Invalid API key")
 		return
@@ -299,7 +299,7 @@ func (h *GatewayHandler) GeminiV1BetaModels(c *gin.Context) {
 			defer cancel()
 			if err := h.gatewayService.RecordUsage(ctx, &service.RecordUsageInput{
 				Result:       result,
-				ApiKey:       apiKey,
+				APIKey:       apiKey,
 				User:         apiKey.User,
 				Account:      usedAccount,
 				Subscription: subscription,