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/domain_constants.go

@@ -258,6 +258,9 @@ const (
 	// Account Quota Notification
 	SettingKeyAccountQuotaNotifyEnabled = "account_quota_notify_enabled" // 全局开关
 	SettingKeyAccountQuotaNotifyEmails  = "account_quota_notify_emails"  // 管理员通知邮箱列表（JSON 数组）
+
+	// Web Search Emulation
+	SettingKeyWebSearchEmulationConfig = "web_search_emulation_config" // JSON 配置
 )
 
 // AdminAPIKeyPrefix is the prefix for admin API keys (distinct from user "sk-" keys).