fix: address audit findings - cache sync, validation, consistency

- clearCreditsExhausted: sync Redis scheduler cache after DB update
- Image billing mode UI: write to per_request_price instead of image_output_price
- OpenAI RecordUsage: use BillingModelSourceRequested constant, add s.cfg nil guard
- Fix i18n key path: admin.channels.perRequestPriceRequired → admin.channels.form.perRequestPriceRequired

backend/internal/service/antigravity_credits_overages.go

@@ -140,6 +140,10 @@ func (s *AntigravityGatewayService) clearCreditsExhausted(ctx context.Context, a
 	}); err != nil {
 		logger.LegacyPrintf("service.antigravity_gateway", "clear credits exhausted failed: account=%d err=%v", account.ID, err)
 	}
+	// 同步更新 Redis 调度快照，避免其他节点/请求延迟感知
+	if s.schedulerSnapshot != nil {
+		_ = s.schedulerSnapshot.UpdateAccountInCache(ctx, account)
+	}
 }
 
 // classifyAntigravity429 将 Antigravity 的 429 响应归类为配额耗尽、限流或未知。