diff --git a/backend/cmd/server/VERSION b/backend/cmd/server/VERSION
index 0ac4459d..bc34277a 100644
--- a/backend/cmd/server/VERSION
+++ b/backend/cmd/server/VERSION
@@ -1 +1 @@
-0.1.110.47
+0.1.110.48
diff --git a/backend/internal/handler/admin/setting_handler.go b/backend/internal/handler/admin/setting_handler.go
index bc6d183c..0c1606ea 100644
--- a/backend/internal/handler/admin/setting_handler.go
+++ b/backend/internal/handler/admin/setting_handler.go
@@ -5,11 +5,10 @@ import (
 	"encoding/hex"
 	"encoding/json"
 	"fmt"
-	"log"
+	"log/slog"
 	"net/http"
 	"regexp"
 	"strings"
-	"time"
 
 	"github.com/Wei-Shaw/sub2api/internal/config"
 	"github.com/Wei-Shaw/sub2api/internal/handler/dto"
@@ -1120,11 +1119,11 @@ func (h *SettingHandler) auditSettingsUpdate(c *gin.Context, before *service.Sys
 
 	subject, _ := middleware.GetAuthSubjectFromContext(c)
 	role, _ := middleware.GetUserRoleFromContext(c)
-	log.Printf("AUDIT: settings updated at=%s user_id=%d role=%s changed=%v",
-		time.Now().UTC().Format(time.RFC3339),
-		subject.UserID,
-		role,
-		changed,
+	slog.Info("settings updated",
+		"audit", true,
+		"user_id", subject.UserID,
+		"role", role,
+		"changed", changed,
 	)
 }
 
@@ -1358,6 +1357,22 @@ func diffSettings(before *service.SystemSettings, after *service.SystemSettings,
 	if before.EnableCCHSigning != after.EnableCCHSigning {
 		changed = append(changed, "enable_cch_signing")
 	}
+	// Balance & quota notification
+	if before.BalanceLowNotifyEnabled != after.BalanceLowNotifyEnabled {
+		changed = append(changed, "balance_low_notify_enabled")
+	}
+	if before.BalanceLowNotifyThreshold != after.BalanceLowNotifyThreshold {
+		changed = append(changed, "balance_low_notify_threshold")
+	}
+	if before.BalanceLowNotifyRechargeURL != after.BalanceLowNotifyRechargeURL {
+		changed = append(changed, "balance_low_notify_recharge_url")
+	}
+	if before.AccountQuotaNotifyEnabled != after.AccountQuotaNotifyEnabled {
+		changed = append(changed, "account_quota_notify_enabled")
+	}
+	if !equalNotifyEmailEntries(before.AccountQuotaNotifyEmails, after.AccountQuotaNotifyEmails) {
+		changed = append(changed, "account_quota_notify_emails")
+	}
 	return changed
 }
 
@@ -1414,6 +1429,18 @@ func equalIntSlice(a, b []int) bool {
 	return true
 }
 
+func equalNotifyEmailEntries(a, b []service.NotifyEmailEntry) bool {
+	if len(a) != len(b) {
+		return false
+	}
+	for i := range a {
+		if a[i].Email != b[i].Email || a[i].Verified != b[i].Verified || a[i].Disabled != b[i].Disabled {
+			return false
+		}
+	}
+	return true
+}
+
 // TestSMTPRequest 测试SMTP连接请求
 type TestSMTPRequest struct {
 	SMTPHost     string `json:"smtp_host"`
diff --git a/frontend/src/components/account/QuotaNotifyToggle.vue b/frontend/src/components/account/QuotaNotifyToggle.vue
index 0548f661..954d0929 100644
--- a/frontend/src/components/account/QuotaNotifyToggle.vue
+++ b/frontend/src/components/account/QuotaNotifyToggle.vue
@@ -1,4 +1,6 @@
 <script setup lang="ts">
+import { QUOTA_THRESHOLD_TYPE_FIXED, QUOTA_THRESHOLD_TYPE_PERCENTAGE } from '@/constants/account'
+
 defineProps<{
   enabled: boolean | null
   threshold: number | null
@@ -35,17 +37,17 @@ const emit = defineEmits<{
         @input="emit('update:threshold', parseFloat(($event.target as HTMLInputElement).value) || null)"
         type="number"
         min="0"
-        :max="thresholdType === 'percentage' ? 100 : undefined"
-        :step="thresholdType === 'percentage' ? 1 : 0.01"
+        :max="thresholdType === QUOTA_THRESHOLD_TYPE_PERCENTAGE ? 100 : undefined"
+        :step="thresholdType === QUOTA_THRESHOLD_TYPE_PERCENTAGE ? 1 : 0.01"
         class="input py-1 text-sm flex-1 min-w-0"
       />
       <select
-        :value="thresholdType || 'fixed'"
+        :value="thresholdType || QUOTA_THRESHOLD_TYPE_FIXED"
         @change="emit('update:thresholdType', ($event.target as HTMLSelectElement).value)"
         class="input py-1 text-xs w-[4.5rem] flex-shrink-0 text-center"
       >
-        <option value="fixed">$</option>
-        <option value="percentage">%</option>
+        <option :value="QUOTA_THRESHOLD_TYPE_FIXED">$</option>
+        <option :value="QUOTA_THRESHOLD_TYPE_PERCENTAGE">%</option>
       </select>
     </template>
   </div>
diff --git a/frontend/src/components/admin/usage/UsageTable.vue b/frontend/src/components/admin/usage/UsageTable.vue
index 441e89e0..55d4f84b 100644
--- a/frontend/src/components/admin/usage/UsageTable.vue
+++ b/frontend/src/components/admin/usage/UsageTable.vue
@@ -93,7 +93,7 @@
 
         <template #cell-tokens="{ row }">
           <!-- 图片生成请求（仅按次计费时显示图片格式） -->
-          <div v-if="row.image_count > 0 && row.billing_mode === 'image'" class="flex items-center gap-1.5">
+          <div v-if="row.image_count > 0 && row.billing_mode === BILLING_MODE_IMAGE" class="flex items-center gap-1.5">
             <svg class="h-4 w-4 text-indigo-500" fill="none" stroke="currentColor" viewBox="0 0 24 24">
               <path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M4 16l4.586-4.586a2 2 0 012.828 0L16 16m-2-2l1.586-1.586a2 2 0 012.828 0L20 14m-6-6h.01M6 20h12a2 2 0 002-2V6a2 2 0 00-2-2H6a2 2 0 00-2 2v12a2 2 0 002 2z" />
             </svg>
@@ -280,7 +280,7 @@
               <span class="font-medium text-white">${{ tooltipData.output_cost.toFixed(6) }}</span>
             </div>
             <!-- Token billing: show unit prices per 1M tokens -->
-            <template v-if="!tooltipData?.billing_mode || tooltipData.billing_mode === 'token'">
+            <template v-if="!tooltipData?.billing_mode || tooltipData.billing_mode === BILLING_MODE_TOKEN">
               <div v-if="tooltipData && tooltipData.input_tokens > 0" class="flex items-center justify-between gap-4">
                 <span class="text-gray-400">{{ t('usage.inputTokenPrice') }}</span>
                 <span class="font-medium text-sky-300">{{ formatTokenPricePerMillion(tooltipData.input_cost, tooltipData.input_tokens) }} {{ t('usage.perMillionTokens') }}</span>
@@ -292,7 +292,7 @@
             </template>
             <!-- Per-request / image billing: show unit price -->
             <div v-else class="flex items-center justify-between gap-4">
-              <span class="text-gray-400">{{ tooltipData.billing_mode === 'image' ? t('usage.imageUnitPrice') : t('usage.unitPrice') }}</span>
+              <span class="text-gray-400">{{ tooltipData.billing_mode === BILLING_MODE_IMAGE ? t('usage.imageUnitPrice') : t('usage.unitPrice') }}</span>
               <span class="font-medium text-sky-300">${{ tooltipData.total_cost?.toFixed(6) || '0.000000' }}</span>
             </div>
             <div v-if="tooltipData && tooltipData.cache_creation_cost > 0" class="flex items-center justify-between gap-4">
@@ -350,7 +350,7 @@ import { formatCacheTokens, formatMultiplier } from '@/utils/formatters'
 import { formatTokenPricePerMillion } from '@/utils/usagePricing'
 import { getUsageServiceTierLabel } from '@/utils/usageServiceTier'
 import { resolveUsageRequestType } from '@/utils/usageRequestType'
-import { getBillingModeLabel, getBillingModeBadgeClass } from '@/utils/billingMode'
+import { getBillingModeLabel, getBillingModeBadgeClass, BILLING_MODE_TOKEN, BILLING_MODE_IMAGE } from '@/utils/billingMode'
 
 /** Compute the account-billed cost for display: (account_stats_cost ?? total_cost) * rate_multiplier */
 function accountBilled(row: { total_cost?: number | null; account_stats_cost?: number | null; account_rate_multiplier?: number | null }): number {
diff --git a/frontend/src/constants/account.ts b/frontend/src/constants/account.ts
new file mode 100644
index 00000000..3c151fee
--- /dev/null
+++ b/frontend/src/constants/account.ts
@@ -0,0 +1,10 @@
+/** WebSearch emulation mode values (must match backend WebSearchMode* constants in account.go) */
+export const WEB_SEARCH_MODE_DEFAULT = 'default' as const
+export const WEB_SEARCH_MODE_ENABLED = 'enabled' as const
+export const WEB_SEARCH_MODE_DISABLED = 'disabled' as const
+export type WebSearchMode = typeof WEB_SEARCH_MODE_DEFAULT | typeof WEB_SEARCH_MODE_ENABLED | typeof WEB_SEARCH_MODE_DISABLED
+
+/** Quota notification threshold type values (must match thresholdType* constants in balance_notify_service.go) */
+export const QUOTA_THRESHOLD_TYPE_FIXED = 'fixed' as const
+export const QUOTA_THRESHOLD_TYPE_PERCENTAGE = 'percentage' as const
+export type QuotaThresholdType = typeof QUOTA_THRESHOLD_TYPE_FIXED | typeof QUOTA_THRESHOLD_TYPE_PERCENTAGE