Aggiunge overlay server-side burn-in e stabilizza diretta live.

Tabellone, badge e brand sono ricodificati nel flusso _air via OverlayRelay;
sync punteggio HTTP, volume condiviso rails/sidekiq, qualità 720p migliorata
e badge CONNECTING in ripresa da pausa.

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

backend/app/services/streams/youtube_relay.rb

@@ -0,0 +1,103 @@
+module Streams
+  # Relay continuo verso YouTube: legge l'uscita MediaMTX del path (camera o slate alwaysAvailable)
+  # senza interrompere FLV quando il telefono si mette in pausa o perde rete.
+  class YoutubeRelay
+    class Error < StandardError; end
+
+    REDIS_KEY = "youtube_relay:pid:%s"
+
+    class << self
+      def start(session)
+        return unless session.platform == "youtube"
+        return if session.stream_key.blank?
+        return if session.terminal?
+
+        stop(session) if pid_for(session.id).present?
+
+        log_path = log_file(session)
+        FileUtils.mkdir_p(File.dirname(log_path))
+        intake = mediamtx_intake_url(session)
+        output = "rtmp://a.rtmp.youtube.com/live2/#{session.stream_key}"
+
+        pid = Process.spawn(
+          "ffmpeg", "-nostdin", "-hide_banner", "-loglevel", "warning",
+          "-rw_timeout", "15000000",
+          "-i", intake,
+          "-c", "copy",
+          "-f", "flv",
+          output,
+          %i[out err] => log_path,
+          pgroup: true
+        )
+        Process.detach(pid)
+        store_pid(session.id, pid)
+        Rails.logger.info("[YoutubeRelay] started pid=#{pid} session=#{session.id}")
+        pid
+      rescue Errno::ENOENT => e
+        raise Error, "ffmpeg non disponibile: #{e.message}"
+      end
+
+      def stop(session)
+        pid = pid_for(session.id)
+        return false if pid.blank?
+
+        terminate_pid(pid)
+        clear_pid(session.id)
+        Rails.logger.info("[YoutubeRelay] stopped pid=#{pid} session=#{session.id}")
+        true
+      end
+
+      def running?(session_id)
+        pid = pid_for(session_id)
+        pid.present? && process_alive?(pid)
+      end
+
+      private
+
+      def mediamtx_intake_url(session)
+        base = ENV.fetch("MEDIAMTX_INTERNAL_RTMP_URL", "rtmp://mediamtx:1935")
+        "#{base.chomp('/')}/#{session.mediamtx_overlay_path_name}"
+      end
+
+      def log_file(session)
+        Rails.root.join("log", "youtube_relay_#{session.id}.log").to_s
+      end
+
+      def redis
+        @redis ||= Redis.new(url: ENV.fetch("REDIS_URL", "redis://localhost:6379/0"))
+      end
+
+      def store_pid(session_id, pid)
+        redis.set(format(REDIS_KEY, session_id), pid, ex: 48.hours.to_i)
+      end
+
+      def clear_pid(session_id)
+        redis.del(format(REDIS_KEY, session_id))
+      end
+
+      def pid_for(session_id)
+        redis.get(format(REDIS_KEY, session_id))
+      end
+
+      def process_alive?(pid)
+        Process.kill(0, pid.to_i)
+        true
+      rescue Errno::ESRCH, Errno::EPERM
+        false
+      end
+
+      def terminate_pid(pid)
+        Process.kill("TERM", -pid.to_i)
+        sleep 0.5
+      rescue Errno::ESRCH
+        nil
+      else
+        begin
+          Process.kill("KILL", -pid.to_i)
+        rescue Errno::ESRCH
+          nil
+        end
+      end
+    end
+  end
+end