Add show management and going-live controls to admin

- admin/shows.php: list shows
- admin/show_edit.php: add, edit and delete shows, with validated
  background image uploads saved under generated names
- admin/index.php: queue the next show, switch the live show, or clear
  the queue
- includes/broadcasts.php: setBroadcastShow() and clearQueuedShow(),
  shared by the dashboard and cli/set_show.php
- includes/admin_layout.php: flash messages and Shows nav link

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

cli/set_show.php

@@ -33,12 +33,8 @@ if (!$show) {
     exit(1);
 }
 
-$live = liveBroadcast($db);
-if ($live !== null) {
-    $db->prepare('UPDATE broadcasts SET show_id = ? WHERE id = ?')->execute([$show['id'], $live['id']]);
+if (setBroadcastShow($db, (int)$show['id']) === 'live') {
     echo "Live broadcast is now {$show['name']}.\n";
 } else {
-    $db->exec('DELETE FROM broadcasts WHERE started_at IS NULL');
-    $db->prepare('INSERT INTO broadcasts (show_id) VALUES (?)')->execute([$show['id']]);
     echo "{$show['name']} is queued for the next broadcast.\n";
 }