fix: round-2 audit fixes — security, code quality, and UI improvements

Security (HIGH):
- Normalize all Redis cache keys to lowercase (verifyCode, passwordReset)
- Fix verify code TTL renewal on failed attempts: use remaining TTL via
  ExpiresAt field instead of resetting to full 15-minute window
- Add 3 missing fields to diffSettings audit log (promo_code, invitation_code,
  custom_endpoints)

Code quality (MEDIUM):
- Extract filterVerifiedEmails shared helper (balance_notify_service.go)
- Add Pricing array non-empty validation for channel pricing rules
- Add platform token semantics comment in gateway_service.go
- Complete validatePlanPatch test coverage (+10 test cases)
- Replace string types with QuotaThresholdType/QuotaResetMode across frontend
- Remove duplicate getPlatformTextColor/getRateBadgeClass in ChannelsView
- Return EMAIL_NOT_FOUND error on RemoveNotifyEmail miss

UI improvements:
- Reorder cost tooltip: user billing above separator, account billing below
- Add NaN guard to accountBilled function
- Move timezone selector inline into reset-mode row (no longer standalone)

frontend/src/components/admin/usage/UsageTable.vue

@@ -313,10 +313,6 @@
             <span class="text-gray-400">{{ t('usage.rate') }}</span>
             <span class="font-semibold text-blue-400">{{ formatMultiplier(tooltipData?.rate_multiplier || 1) }}x</span>
           </div>
-          <div class="flex items-center justify-between gap-6">
-            <span class="text-gray-400">{{ t('usage.accountMultiplier') }}</span>
-            <span class="font-semibold text-blue-400">{{ formatMultiplier(tooltipData?.account_rate_multiplier ?? 1) }}x</span>
-          </div>
           <div class="flex items-center justify-between gap-6">
             <span class="text-gray-400">{{ t('usage.original') }}</span>
             <span class="font-medium text-white">${{ tooltipData?.total_cost?.toFixed(6) || '0.000000' }}</span>
@@ -325,7 +321,12 @@
             <span class="text-gray-400">{{ t('usage.userBilled') }}</span>
             <span class="font-semibold text-green-400">${{ tooltipData?.actual_cost?.toFixed(6) || '0.000000' }}</span>
           </div>
+          <!-- Account billing (separated from user billing) -->
           <div class="flex items-center justify-between gap-6 border-t border-gray-700 pt-1.5">
+            <span class="text-gray-400">{{ t('usage.accountMultiplier') }}</span>
+            <span class="font-semibold text-blue-400">{{ formatMultiplier(tooltipData?.account_rate_multiplier ?? 1) }}x</span>
+          </div>
+          <div class="flex items-center justify-between gap-6">
             <span class="text-gray-400">{{ t('usage.accountBilled') }}</span>
             <span class="font-semibold text-green-400">
               ${{ accountBilled({
@@ -355,7 +356,8 @@ import { getBillingModeLabel, getBillingModeBadgeClass, BILLING_MODE_TOKEN, BILL
 /** 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 {
   const base = row.account_stats_cost != null ? row.account_stats_cost : (row.total_cost ?? 0)
-  return base * (row.account_rate_multiplier ?? 1)
+  const result = base * (row.account_rate_multiplier ?? 1)
+  return Number.isNaN(result) ? 0 : result
 }
 
 import DataTable from '@/components/common/DataTable.vue'