refactor: unify post-usage billing logic and fix account quota calculation

- Extract postUsageBilling() to consolidate billing logic across
  GatewayService.RecordUsage, RecordUsageWithLongContext, and
  OpenAIGatewayService.RecordUsage, eliminating ~120 lines of
  duplicated code
- Fix account quota to use TotalCost × accountRateMultiplier
  (was using raw TotalCost, inconsistent with account cost stats)
- Fix RecordUsageWithLongContext API Key quota only updating in
  balance mode (now updates regardless of billing type)
- Fix WebSocket client disconnect detection on Windows by adding
  "an established connection was aborted" to known disconnect errors

backend/internal/service/openai_ws_forwarder.go

@@ -864,7 +864,8 @@ func isOpenAIWSClientDisconnectError(err error) bool {
 		strings.Contains(message, "unexpected eof") ||
 		strings.Contains(message, "use of closed network connection") ||
 		strings.Contains(message, "connection reset by peer") ||
-		strings.Contains(message, "broken pipe")
+		strings.Contains(message, "broken pipe") ||
+		strings.Contains(message, "an established connection was aborted")
 }
 
 func classifyOpenAIWSReadFallbackReason(err error) string {