Address Brother QL printer by URL path instead of session cookie

Drops /brother/select and the session-based "active printer" state:
the printer key now lives in the path (/brother/<printer>/*), so the
proxy is stateless and any path under it -- including brother_ql_web's
API, not just its UI -- is reachable without a prior "activation" step.

app.rb

@@ -89,9 +89,10 @@ helpers do
   end
 
   # Transparently reverse-proxies the current request to one of the
-  # brother_ql_web instances, so all three printers are reachable through
-  # this single sinatra port instead of their own dedicated ports.
-  def proxy_to_brother(port, path)
+  # brother_ql_web instances, so all three printers (web UI and API alike)
+  # are reachable through this single sinatra port instead of their own
+  # dedicated ports.
+  def proxy_to_brother(port, prefix, path)
     uri = URI("http://127.0.0.1:#{port}#{path}")
     uri.query = request.query_string unless request.query_string.empty?
 
@@ -103,7 +104,7 @@ helpers do
       proxied[header] = value
     end
     proxied['Content-Type'] = request.content_type if request.content_type
-    proxied['X-Forwarded-Prefix'] = '/brother'
+    proxied['X-Forwarded-Prefix'] = prefix
     proxied['X-Forwarded-Host'] = request.host
     proxied.body = request.body.read unless request.request_method == 'GET'
 
@@ -188,23 +189,14 @@ end
 
 get '/brother' do
   @printers = BROTHER_PRINTERS
-  @selected = session[:brother_printer]
   erb :brother_picker
 end
 
-post '/brother/select' do
-  key = params[:printer]
-  halt 400, 'unknown printer' unless BROTHER_PRINTERS.key?(key)
-
-  session[:brother_printer] = key
-  redirect '/brother/'
-end
-
 %w[get post put delete].each do |verb|
-  send(verb, '/brother/*') do
-    key = session[:brother_printer]
-    redirect '/brother' unless key
+  send(verb, '/brother/:printer/*') do
+    printer = BROTHER_PRINTERS[params[:printer]]
+    halt 404, 'unknown printer' unless printer
 
-    proxy_to_brother(BROTHER_PRINTERS[key][:port], '/' + params[:splat].first)
+    proxy_to_brother(printer[:port], "/brother/#{params[:printer]}", '/' + params[:splat].first)
   end
 end