Allow comments for LNURL-PAY invoices

Allows senders to add a short message to payments, which will be stored
as invoice memo by LND/LndHub.

app/controllers/lnurlpay_controller.rb

@@ -3,6 +3,7 @@ class LnurlpayController < ApplicationController
 
   MIN_SATS = 100
   MAX_SATS = 1_000_000
+  MAX_COMMENT_CHARS = 100
 
   def index
     render json: {
@@ -12,22 +13,29 @@ class LnurlpayController < ApplicationController
       maxSendable: MAX_SATS * 1000, # msat
       minSendable: MIN_SATS * 1000, # msat
       metadata: metadata(@user.address),
-      commentAllowed: 0
+      commentAllowed: MAX_COMMENT_CHARS
     }
   end
 
   def invoice
     amount = params[:amount].to_i / 1000 # msats
     address = params[:address]
+    comment = params[:comment]
 
     if !valid_amount?(amount)
       render json: { status: "ERROR", reason: "Invalid amount" }
       return
     end
 
+    if !valid_comment?(comment)
+      render json: { status: "ERROR", reason: "Comment too long" }
+      return
+    end
+
     payment_request = @user.ln_create_invoice({
       amount: amount, # we create invoices in sats
-      description_hash: Digest::SHA2.hexdigest(metadata(address))
+      description_hash: Digest::SHA2.hexdigest(metadata(address)),
+      memo: comment
     })
 
     render json: {
@@ -57,4 +65,8 @@ class LnurlpayController < ApplicationController
     amount_in_sats <= MAX_SATS && amount_in_sats >= MIN_SATS
   end
 
+  def valid_comment?(comment)
+    comment.length <= MAX_COMMENT_CHARS
+  end
+
 end