feat: add comment support for Lightning payments in BTCPay and Strike services

- Enhanced the sendPaymentWithAddress method in BTCPayService and StrikeService to accept an optional comment parameter.
- Updated resolveLightningAddress to include the comment in the callback URL if supported by the LNURL-pay endpoint.
- Modified PaymentService to construct a descriptive comment for Lightning invoices, improving clarity for users.

These changes enhance the payment experience by allowing users to include contextual information with their transactions.

backend/src/payment/providers/services/btcpay.service.ts

@@ -345,12 +345,13 @@ export class BTCPayService implements LightningService {
   async sendPaymentWithAddress(
     address: string,
     payment: Payment,
+    comment?: string,
   ): Promise<LightningPaymentDTO> {
     try {
       const sats = Math.floor(payment.milisatsAmount / 1000);
 
       // Resolve the Lightning address to a BOLT11 invoice
-      const bolt11 = await this.resolveLightningAddress(address, sats);
+      const bolt11 = await this.resolveLightningAddress(address, sats, comment);
 
       // Pay the BOLT11 via BTCPay's internal Lightning node
       const payUrl = this.storeUrl('/lightning/BTC/invoices/pay');
@@ -578,6 +579,7 @@ export class BTCPayService implements LightningService {
   private async resolveLightningAddress(
     address: string,
     amountSats: number,
+    comment?: string,
   ): Promise<string> {
     const [username, domain] = address.split('@');
     if (!username || !domain) {
@@ -606,6 +608,16 @@ export class BTCPayService implements LightningService {
     const callbackUrl = new URL(lnurlData.callback);
     callbackUrl.searchParams.set('amount', amountMillisats.toString());
 
+    // Include a descriptive comment if the endpoint supports it.
+    // LNURL-pay endpoints advertise commentAllowed (max char length).
+    const commentAllowed = lnurlData.commentAllowed || 0;
+    if (comment && commentAllowed > 0) {
+      callbackUrl.searchParams.set(
+        'comment',
+        comment.slice(0, commentAllowed),
+      );
+    }
+
     const { data: invoiceData } = await axios.get(callbackUrl.toString(), { timeout: 10_000 });
 
     if (invoiceData.status === 'ERROR') {