diff --git a/backend/internal/handler/admin/account_handler.go b/backend/internal/handler/admin/account_handler.go
index 58715706..78b71431 100644
--- a/backend/internal/handler/admin/account_handler.go
+++ b/backend/internal/handler/admin/account_handler.go
@@ -1023,6 +1023,10 @@ func (h *AccountHandler) RefreshTier(c *gin.Context) {
 	}
 
 	tierID, storageInfo, err := h.geminiOAuthService.FetchGoogleOneTier(c.Request.Context(), accessToken, proxyURL)
+	if err != nil {
+		response.ErrorFrom(c, err)
+		return
+	}
 
 	if account.Extra == nil {
 		account.Extra = make(map[string]any)
@@ -1044,10 +1048,10 @@ func (h *AccountHandler) RefreshTier(c *gin.Context) {
 	}
 
 	response.Success(c, gin.H{
-		"tier_id":              tierID,
-		"drive_storage_limit":  account.Extra["drive_storage_limit"],
-		"drive_storage_usage":  account.Extra["drive_storage_usage"],
-		"updated_at":           account.Extra["drive_tier_updated_at"],
+		"tier_id":             tierID,
+		"drive_storage_limit": account.Extra["drive_storage_limit"],
+		"drive_storage_usage": account.Extra["drive_storage_usage"],
+		"updated_at":          account.Extra["drive_tier_updated_at"],
 	})
 }
 
diff --git a/backend/internal/pkg/geminicli/drive_client.go b/backend/internal/pkg/geminicli/drive_client.go
index 5a959fac..79d6835f 100644
--- a/backend/internal/pkg/geminicli/drive_client.go
+++ b/backend/internal/pkg/geminicli/drive_client.go
@@ -22,17 +22,11 @@ type DriveClient interface {
 	GetStorageQuota(ctx context.Context, accessToken, proxyURL string) (*DriveStorageInfo, error)
 }
 
-type driveClient struct {
-	httpClient *http.Client
-}
+type driveClient struct{}
 
 // NewDriveClient creates a new Drive API client
 func NewDriveClient() DriveClient {
-	return &driveClient{
-		httpClient: &http.Client{
-			Timeout: 10 * time.Second,
-		},
-	}
+	return &driveClient{}
 }
 
 // GetStorageQuota fetches storage quota from Google Drive API
@@ -71,7 +65,7 @@ func (c *driveClient) GetStorageQuota(ctx context.Context, accessToken, proxyURL
 
 		// Rate limit - retry with exponential backoff
 		if resp.StatusCode == http.StatusTooManyRequests && attempt < maxRetries-1 {
-			resp.Body.Close()
+			_ = resp.Body.Close()
 			backoff := time.Duration(1<<uint(attempt)) * time.Second // 1s, 2s, 4s
 			time.Sleep(backoff)
 			continue
@@ -79,11 +73,11 @@ func (c *driveClient) GetStorageQuota(ctx context.Context, accessToken, proxyURL
 
 		// Other errors - return immediately
 		body, _ := io.ReadAll(resp.Body)
-		resp.Body.Close()
-		return nil, fmt.Errorf("Drive API error (status %d): %s", resp.StatusCode, string(body))
+		_ = resp.Body.Close()
+		return nil, fmt.Errorf("drive API error (status %d): %s", resp.StatusCode, string(body))
 	}
 
-	defer resp.Body.Close()
+	defer func() { _ = resp.Body.Close() }()
 
 	// Parse response
 	var result struct {
@@ -100,10 +94,10 @@ func (c *driveClient) GetStorageQuota(ctx context.Context, accessToken, proxyURL
 	// Parse limit and usage (handle both string and number formats)
 	var limit, usage int64
 	if result.StorageQuota.Limit != "" {
-		fmt.Sscanf(result.StorageQuota.Limit, "%d", &limit)
+		_, _ = fmt.Sscanf(result.StorageQuota.Limit, "%d", &limit)
 	}
 	if result.StorageQuota.Usage != "" {
-		fmt.Sscanf(result.StorageQuota.Usage, "%d", &usage)
+		_, _ = fmt.Sscanf(result.StorageQuota.Usage, "%d", &usage)
 	}
 
 	return &DriveStorageInfo{
diff --git a/backend/internal/service/gemini_oauth_service.go b/backend/internal/service/gemini_oauth_service.go
index ba82545b..a6b63924 100644
--- a/backend/internal/service/gemini_oauth_service.go
+++ b/backend/internal/service/gemini_oauth_service.go
@@ -18,11 +18,11 @@ import (
 )
 
 const (
-	TierAIPremium         = "AI_PREMIUM"
-	TierGoogleOneStandard = "GOOGLE_ONE_STANDARD"
-	TierGoogleOneBasic    = "GOOGLE_ONE_BASIC"
-	TierFree              = "FREE"
-	TierGoogleOneUnknown  = "GOOGLE_ONE_UNKNOWN"
+	TierAIPremium          = "AI_PREMIUM"
+	TierGoogleOneStandard  = "GOOGLE_ONE_STANDARD"
+	TierGoogleOneBasic     = "GOOGLE_ONE_BASIC"
+	TierFree               = "FREE"
+	TierGoogleOneUnknown   = "GOOGLE_ONE_UNKNOWN"
 	TierGoogleOneUnlimited = "GOOGLE_ONE_UNLIMITED"
 )
 
@@ -340,7 +340,8 @@ func (s *GeminiOAuthService) ExchangeCode(ctx context.Context, input *GeminiExch
 	// 对于 code_assist 模式，project_id 是必需的，需要调用 Code Assist API
 	// 对于 google_one 模式，使用个人 Google 账号，不需要 project_id，配额由 Google 网关自动识别
 	// 对于 ai_studio 模式，project_id 是可选的（不影响使用 AI Studio API）
-	if oauthType == "code_assist" {
+	switch oauthType {
+	case "code_assist":
 		if projectID == "" {
 			var err error
 			projectID, tierID, err = s.fetchProjectID(ctx, tokenResp.AccessToken, proxyURL)
@@ -364,7 +365,7 @@ func (s *GeminiOAuthService) ExchangeCode(ctx context.Context, input *GeminiExch
 		if tierID == "" {
 			tierID = "LEGACY"
 		}
-	} else if oauthType == "google_one" {
+	case "google_one":
 		// Attempt to fetch Drive storage tier
 		tierID, storageInfo, err := s.FetchGoogleOneTier(ctx, tokenResp.AccessToken, proxyURL)
 		if err != nil {
@@ -523,7 +524,8 @@ func (s *GeminiOAuthService) RefreshAccountToken(ctx context.Context, account *A
 
 	// For Code Assist, project_id is required. Auto-detect if missing.
 	// For AI Studio OAuth, project_id is optional and should not block refresh.
-	if oauthType == "code_assist" {
+	switch oauthType {
+	case "code_assist":
 		// 先设置默认值或保留旧值，确保 tier_id 始终有值
 		if existingTierID != "" {
 			tokenInfo.TierID = existingTierID
@@ -551,7 +553,7 @@ func (s *GeminiOAuthService) RefreshAccountToken(ctx context.Context, account *A
 		if strings.TrimSpace(tokenInfo.ProjectID) == "" {
 			return nil, fmt.Errorf("failed to auto-detect project_id: empty result")
 		}
-	} else if oauthType == "google_one" {
+	case "google_one":
 		// Check if tier cache is stale (> 24 hours)
 		needsRefresh := true
 		if account.Extra != nil {