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/settings_view.go

@@ -107,6 +107,9 @@ type SystemSettings struct {
 	EnableMetadataPassthrough    bool // 是否透传客户端原始 metadata（默认 false）
 	EnableCCHSigning             bool // 是否对 billing header cch 进行签名（默认 false）
 
+	// Web Search Emulation
+	WebSearchEmulationEnabled bool // 是否启用 web search 模拟
+
 	// Balance low notification
 	BalanceLowNotifyEnabled     bool
 	BalanceLowNotifyThreshold   float64