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/base.rb

@@ -0,0 +1,21 @@
+# frozen_string_literal: true
+
+module Streams
+  module DnsProviders
+    class Error < StandardError; end
+
+    class Base
+      def upsert_a(name, ip)
+        raise NotImplementedError
+      end
+
+      def delete_a(name)
+        raise NotImplementedError
+      end
+
+      def resolve(name)
+        raise NotImplementedError
+      end
+    end
+  end
+end