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/mediamtx/client.rb

@@ -4,7 +4,12 @@ module Mediamtx
   class Client
     class Error < StandardError; end
 
+    def self.for_session(session)
+      new(base_url: session.mediamtx_api_base_url)
+    end
+
     def initialize(base_url: MatchLiveTv.mediamtx_api_url)
+      @base_url = base_url
       @conn = Faraday.new(url: base_url) do |f|
         f.request :json
         f.response :json
@@ -12,6 +17,8 @@ module Mediamtx
       end
     end
 
+    attr_reader :base_url
+
     def create_path(session)
       path = session.mediamtx_path_name
       # record: false finché non c'è publisher — con alwaysAvailable MediaMTX registrerebbe