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>

admin/shows.php

@@ -0,0 +1,46 @@
+<?php
+require __DIR__ . '/../includes/auth.php';
+require __DIR__ . '/../includes/admin_layout.php';
+
+$admin = requireAdmin();
+$shows = getDB()->query('SELECT id, slug, name, background_image FROM shows ORDER BY name')->fetchAll();
+
+adminHeader('Shows', $admin);
+?>
+				<ul class="actions">
+					<li><a href="show_edit.php" class="button primary small">Add a show</a></li>
+				</ul>
+
+				<div class="table-wrapper">
+					<table>
+						<thead>
+							<tr>
+								<th>Background</th>
+								<th>Name</th>
+								<th>Slug</th>
+								<th></th>
+							</tr>
+						</thead>
+						<tbody>
+<?php foreach ($shows as $show): ?>
+							<tr>
+								<td>
+<?php if ($show['background_image']): ?>
+									<img src="../images/backgrounds/<?= e(rawurlencode($show['background_image'])) ?>" alt="" style="width:80px;height:50px;object-fit:cover;border-radius:4px;display:block;">
+<?php else: ?>
+									<em>Default</em>
+<?php endif; ?>
+								</td>
+								<td><?= e($show['name']) ?></td>
+								<td><code><?= e($show['slug']) ?></code></td>
+								<td><a href="show_edit.php?id=<?= (int)$show['id'] ?>">Edit</a></td>
+							</tr>
+<?php endforeach; ?>
+<?php if (!$shows): ?>
+							<tr><td colspan="4">No shows yet.</td></tr>
+<?php endif; ?>
+						</tbody>
+					</table>
+				</div>
+<?php
+adminFooter();