feat: invoice payment flow for paid relays

backend/src/billing.rs

@@ -599,6 +599,62 @@ impl Billing {
         Ok(invoice_response.invoice)
     }
 
+    pub async fn pay_outstanding_nwc_invoices(
+        &self,
+        tenant: &crate::models::Tenant,
+    ) -> Result<()> {
+        if tenant.nwc_url.is_empty() {
+            return Ok(());
+        }
+
+        let invoices = self
+            .stripe_list_invoices(&tenant.stripe_customer_id)
+            .await?;
+        let invoices_arr = invoices.as_array().cloned().unwrap_or_default();
+
+        for invoice in &invoices_arr {
+            let status = invoice["status"].as_str().unwrap_or_default();
+            let amount_due = invoice["amount_due"].as_i64().unwrap_or(0);
+            let invoice_id = invoice["id"].as_str().unwrap_or_default();
+            let currency = invoice["currency"].as_str().unwrap_or("usd");
+
+            if status != "open" || amount_due == 0 || invoice_id.is_empty() {
+                continue;
+            }
+
+            match self
+                .nwc_pay_invoice(amount_due, currency, &tenant.nwc_url)
+                .await
+            {
+                Ok(()) => {
+                    if let Err(e) = self.stripe_pay_invoice_out_of_band(invoice_id).await {
+                        tracing::error!(
+                            error = %e,
+                            invoice_id,
+                            "failed to mark invoice paid out of band"
+                        );
+                    } else {
+                        let _ = self.command.clear_tenant_nwc_error(&tenant.pubkey).await;
+                    }
+                }
+                Err(e) => {
+                    let error_msg = format!("{e}");
+                    tracing::error!(
+                        error = %e,
+                        invoice_id,
+                        "nwc payment failed for outstanding invoice"
+                    );
+                    let _ = self
+                        .command
+                        .set_tenant_nwc_error(&tenant.pubkey, &error_msg)
+                        .await;
+                }
+            }
+        }
+
+        Ok(())
+    }
+
     pub async fn stripe_create_portal_session(&self, customer_id: &str) -> Result<String> {
         let resp = self
             .http