fix: resolve cherry-pick conflicts and restore compilation

- Restore gateway_cache.go to upstream (no lua embeds)
- Restore payment_order.go to upstream (use out_trade_no lookup)
- Restore payment_fulfillment.go to upstream (same reason)
- Add FeaturesConfig field and IsWebSearchEmulationEnabled to Channel
- Add applyAccountStatsCost wrapper function
- Add SettingKeyWebSearchEmulationConfig constant
- Add WebSearchEmulationEnabled to SystemSettings
- Add notify code rate limiting methods to EmailCache interface
- Remove AllowUserRefund references (ent schema not present)
- Fix duplicate import in payment_handler.go
- Fix wire_gen.go argument mismatches

backend/internal/service/account_stats_pricing.go

@@ -227,3 +227,24 @@ func calculateTokenStatsCost(pricing *ChannelModelPricing, tokens UsageTokens) *
 	}
 	return &cost
 }
+
+// applyAccountStatsCost resolves the account stats cost for a usage log entry.
+// It resolves the upstream model (falling back to the requested model) and calls
+// the 4-level priority chain via resolveAccountStatsCost.
+func applyAccountStatsCost(
+	ctx context.Context,
+	usageLog *UsageLog,
+	cs *ChannelService, bs *BillingService,
+	accountID int64, groupID int64,
+	upstreamModel, requestedModel string,
+	tokens UsageTokens,
+	totalCost float64,
+) {
+	model := upstreamModel
+	if model == "" {
+		model = requestedModel
+	}
+	usageLog.AccountStatsCost = resolveAccountStatsCost(
+		ctx, cs, bs, accountID, groupID, model, tokens, 1, totalCost,
+	)
+}