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)

backend/internal/repository/email_cache.go

@@ -20,8 +20,9 @@ const (
 )
 
 // verifyCodeKey generates the Redis key for email verification code.
+// Email is lowercased for case-insensitive consistency.
 func verifyCodeKey(email string) string {
-	return verifyCodeKeyPrefix + email
+	return verifyCodeKeyPrefix + strings.ToLower(email)
 }
 
 // notifyVerifyKey generates the Redis key for notify email verification code.
@@ -33,12 +34,12 @@ func notifyVerifyKey(email string) string {
 
 // passwordResetKey generates the Redis key for password reset token.
 func passwordResetKey(email string) string {
-	return passwordResetKeyPrefix + email
+	return passwordResetKeyPrefix + strings.ToLower(email)
 }
 
 // passwordResetSentAtKey generates the Redis key for password reset email sent timestamp.
 func passwordResetSentAtKey(email string) string {
-	return passwordResetSentAtKeyPrefix + email
+	return passwordResetSentAtKeyPrefix + strings.ToLower(email)
 }
 
 type emailCache struct {