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/cloud_providers/base.rb

@@ -0,0 +1,35 @@
+# frozen_string_literal: true
+
+module Streams
+  module CloudProviders
+    class Error < StandardError; end
+
+    # Descrittore restituito da create_node / list.
+    Instance = Struct.new(
+      :id, :name, :public_ip, :private_ip, :status, :raw,
+      keyword_init: true
+    )
+
+    class Base
+      def create_node(name:, labels: {})
+        raise NotImplementedError
+      end
+
+      def destroy_node(instance_id)
+        raise NotImplementedError
+      end
+
+      def list_nodes(labels: {})
+        raise NotImplementedError
+      end
+
+      def wait_until_running(instance_id, timeout: 120)
+        raise NotImplementedError
+      end
+
+      def public_ip(instance_id)
+        raise NotImplementedError
+      end
+    end
+  end
+end