Add Epson TM-T20II receipt printing (title/body/QR, no ESC/POS knowledge needed)

Custom ReceiptBuilder instead of the escpos gem -- its diacritics handling
needed the same manual UTF-8->CP852 encode call anyway, based on earlier
experiments in ~/test/epson/.

app.rb

@@ -3,6 +3,7 @@ require 'sinatra'
 require "sinatra/reloader"
 require 'tmpdir'
 require 'net/http'
+require_relative 'lib/receipt_builder'
 
 set :protection, :except => :frame_options
 set :environment, :production
@@ -21,6 +22,8 @@ PRINTER_DPI = 203 # DatamaxONeil E4204B Mark III native head resolution
 PRINTER_DARKNESS = 8 # lower than factory default to stop thermal dot-gain from fusing barcode bars
 TEST_LABEL_PDF = File.join(File.dirname(__FILE__), 'test-label.pdf')
 
+PRINTER_DEVICE_EPSON = '/dev/usb/lp_epson' # see /etc/udev/rules.d/99-epson.rules
+
 BROTHER_PRINTERS = {
   'ql560'  => { port: 8560, label: 'Brother QL-560' },
   'ql700'  => { port: 8700, label: 'Brother QL-700' },
@@ -88,6 +91,19 @@ helpers do
     File.open(PRINTER_DEVICE, 'wb') { |f| f.write(data) }
   end
 
+  # Builds a simple receipt (bold centered title + left-aligned body,
+  # optional QR code) so callers don't need to know ESC/POS escape codes.
+  def print_receipt(title:, body:, qr: nil)
+    receipt = ReceiptBuilder.new
+    receipt.align(:center).bold.text("#{title}\n").bold(false) unless title.to_s.empty?
+    receipt.align(:left).text("#{body}\n") unless body.to_s.empty?
+    unless qr.to_s.empty?
+      receipt.align(:center).feed(1).qr(qr).feed(1)
+    end
+    receipt.feed(3).cut
+    receipt.print!(PRINTER_DEVICE_EPSON)
+  end
+
   # Transparently reverse-proxies the current request to one of the
   # brother_ql_web instances, so all three printers (web UI and API alike)
   # are reachable through this single sinatra port instead of their own
@@ -187,6 +203,20 @@ post '/test-print' do
   redirect '/upload'
 end
 
+get '/epson' do
+  erb :epson
+end
+
+post '/epson/print' do
+  begin
+    print_receipt(title: params[:title].to_s, body: params[:body].to_s, qr: params[:qr].to_s)
+    flash 'Sent to Epson printer', 'success'
+  rescue => e
+    flash "Epson print failed (#{e.message})", 'danger'
+  end
+  redirect '/epson'
+end
+
 get '/brother' do
   @printers = BROTHER_PRINTERS
   erb :brother_picker