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/account/QuotaNotifyToggle.vue

@@ -1,16 +1,16 @@
 <script setup lang="ts">
-import { QUOTA_THRESHOLD_TYPE_FIXED, QUOTA_THRESHOLD_TYPE_PERCENTAGE } from '@/constants/account'
+import { QUOTA_THRESHOLD_TYPE_FIXED, QUOTA_THRESHOLD_TYPE_PERCENTAGE, type QuotaThresholdType } from '@/constants/account'
 
 defineProps<{
   enabled: boolean | null
   threshold: number | null
-  thresholdType: string | null // "fixed" (default) or "percentage"
+  thresholdType: QuotaThresholdType | null
 }>()
 
 const emit = defineEmits<{
   'update:enabled': [value: boolean | null]
   'update:threshold': [value: number | null]
-  'update:thresholdType': [value: string | null]
+  'update:thresholdType': [value: QuotaThresholdType | null]
 }>()
 </script>
 
@@ -43,7 +43,7 @@ const emit = defineEmits<{
       />
       <select
         :value="thresholdType || QUOTA_THRESHOLD_TYPE_FIXED"
-        @change="emit('update:thresholdType', ($event.target as HTMLSelectElement).value)"
+        @change="emit('update:thresholdType', ($event.target as HTMLSelectElement).value as QuotaThresholdType)"
         class="input py-1 text-xs w-[4.5rem] flex-shrink-0 text-center"
       >
         <option :value="QUOTA_THRESHOLD_TYPE_FIXED">$</option>