fix: audit round-3 — proxy safety, intervals persistence, SMTP timeout, sort fix

- Skip websearch provider when ProxyID is set but proxy not found (prevent
  silent direct connection bypass)
- Fix sortByStableRandomWeight: pair factors with items so sort.Slice swap
  keeps weights aligned
- Allow empty platform in account_stats_pricing_rules (wildcard matching),
  only force anthropic default for main model_pricing
- Add channel_account_stats_pricing_intervals table and repo layer support
  for interval-based pricing in account stats rules
- calculateTokenStatsCost now uses interval pricing when available
- Replace smtp.SendMail/tls.Dial with net.Dialer timeout (10s dial, 20s IO)
  to prevent goroutine leak on SMTP hang
- Fix gofmt formatting issues
- Web Search label: black text with red warning hint

backend/internal/service/balance_notify_service.go

@@ -477,4 +477,3 @@ func (s *BalanceNotifyService) buildQuotaAlertEmailBody(accountID int64, account
 	}
 	return fmt.Sprintf(quotaAlertEmailTemplate, siteName, accountID, accountName, platform, dimLabel, used, limitStr, remaining, thresholdDisplay)
 }
-