Aggiunge registry nodi stream e provisioning Hetzner/lab per lo scale-out.

Prepara l'architettura multi-nodo (MediaMTX+ffmpeg) con assignment URL per sessione, admin di provision/drain e provider Cloud/DNS astratti verso mltv-stream.net.

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/app/services/streams/dns_providers.rb

@@ -0,0 +1,14 @@
+# frozen_string_literal: true
+
+module Streams
+  module DnsProviders
+    def self.build(name = ENV.fetch("STREAM_DNS_PROVIDER", "lab"))
+      case name.to_s
+      when "lab" then Lab.new
+      when "hetzner" then Hetzner.new
+      else
+        raise Error, "STREAM_DNS_PROVIDER sconosciuto: #{name}"
+      end
+    end
+  end
+end