fix: handle Anthropic credit balance exhausted (400) as account error

When an Anthropic API key's credit balance is depleted, the upstream
returns HTTP 400 with message containing "credit balance". Previously,
the 400 handler only checked for "organization has been disabled",
so credit-exhausted accounts kept being scheduled — every request
returned the same error.

Treat this case identically to 402 (Payment Required): call
handleAuthError → SetError to stop scheduling the account until
an admin manually recovers it after topping up credits.

Closes #1586

backend/internal/service/ratelimit_service.go

@@ -142,11 +142,16 @@ func (s *RateLimitService) HandleUpstreamError(ctx context.Context, account *Acc
 
 	switch statusCode {
 	case 400:
-		// 只有当错误信息包含 "organization has been disabled" 时才禁用
+		// "organization has been disabled" → 永久禁用
 		if strings.Contains(strings.ToLower(upstreamMsg), "organization has been disabled") {
 			msg := "Organization disabled (400): " + upstreamMsg
 			s.handleAuthError(ctx, account, msg)
 			shouldDisable = true
+		} else if account.Platform == PlatformAnthropic && strings.Contains(strings.ToLower(upstreamMsg), "credit balance") {
+			// Anthropic API key 余额不足（语义等同 402），停止调度
+			msg := "Credit balance exhausted (400): " + upstreamMsg
+			s.handleAuthError(ctx, account, msg)
+			shouldDisable = true
 		}
 		// 其他 400 错误（如参数问题）不处理，不禁用账号
 	case 401: