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/service/gemini_messages_compat_service.go

@@ -273,7 +273,7 @@ func (s *GeminiMessagesCompatService) SelectAccountForAIStudioEndpoints(ctx cont
 			return 999
 		}
 		switch a.Type {
-		case AccountTypeApiKey:
+		case AccountTypeAPIKey:
 			if strings.TrimSpace(a.GetCredential("api_key")) != "" {
 				return 0
 			}
@@ -351,7 +351,7 @@ func (s *GeminiMessagesCompatService) Forward(ctx context.Context, c *gin.Contex
 
 	originalModel := req.Model
 	mappedModel := req.Model
-	if account.Type == AccountTypeApiKey {
+	if account.Type == AccountTypeAPIKey {
 		mappedModel = account.GetMappedModel(req.Model)
 	}
 
@@ -374,7 +374,7 @@ func (s *GeminiMessagesCompatService) Forward(ctx context.Context, c *gin.Contex
 	}
 
 	switch account.Type {
-	case AccountTypeApiKey:
+	case AccountTypeAPIKey:
 		buildReq = func(ctx context.Context) (*http.Request, string, error) {
 			apiKey := account.GetCredential("api_key")
 			if strings.TrimSpace(apiKey) == "" {
@@ -621,7 +621,7 @@ func (s *GeminiMessagesCompatService) ForwardNative(ctx context.Context, c *gin.
 	}
 
 	mappedModel := originalModel
-	if account.Type == AccountTypeApiKey {
+	if account.Type == AccountTypeAPIKey {
 		mappedModel = account.GetMappedModel(originalModel)
 	}
 
@@ -643,7 +643,7 @@ func (s *GeminiMessagesCompatService) ForwardNative(ctx context.Context, c *gin.
 	var buildReq func(ctx context.Context) (*http.Request, string, error)
 
 	switch account.Type {
-	case AccountTypeApiKey:
+	case AccountTypeAPIKey:
 		buildReq = func(ctx context.Context) (*http.Request, string, error) {
 			apiKey := account.GetCredential("api_key")
 			if strings.TrimSpace(apiKey) == "" {
@@ -1790,7 +1790,7 @@ func (s *GeminiMessagesCompatService) ForwardAIStudioGET(ctx context.Context, ac
 	}
 
 	switch account.Type {
-	case AccountTypeApiKey:
+	case AccountTypeAPIKey:
 		apiKey := strings.TrimSpace(account.GetCredential("api_key"))
 		if apiKey == "" {
 			return nil, errors.New("gemini api_key not configured")