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/email_service.go

@@ -49,6 +49,10 @@ type EmailCache interface {
 	// Returns true if in cooldown period (email was sent recently)
 	IsPasswordResetEmailInCooldown(ctx context.Context, email string) bool
 	SetPasswordResetEmailCooldown(ctx context.Context, email string, ttl time.Duration) error
+
+	// Notify code rate limiting per user
+	IncrNotifyCodeUserRate(ctx context.Context, userID int64, window time.Duration) (int64, error)
+	GetNotifyCodeUserRate(ctx context.Context, userID int64) (int64, error)
 }
 
 // VerificationCodeData represents verification code data