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/payment_config_service.go

@@ -114,7 +114,6 @@ type CreateProviderInstanceRequest struct {
 	SortOrder       int               `json:"sort_order"`
 	Limits          string            `json:"limits"`
 	RefundEnabled   bool              `json:"refund_enabled"`
-	AllowUserRefund bool              `json:"allow_user_refund"`
 }
 
 type UpdateProviderInstanceRequest struct {
@@ -126,7 +125,6 @@ type UpdateProviderInstanceRequest struct {
 	SortOrder       *int              `json:"sort_order"`
 	Limits          *string           `json:"limits"`
 	RefundEnabled   *bool             `json:"refund_enabled"`
-	AllowUserRefund *bool             `json:"allow_user_refund"`
 }
 type CreatePlanRequest struct {
 	GroupID       int64    `json:"group_id"`