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>

includes/broadcasts.php

@@ -43,6 +43,23 @@ function currentShow(PDO $db): ?array {
     return $row ?: null;
 }
 
+// Assigns a show to the live broadcast, or queues it for the next one if nothing is live.
+// Returns 'live' or 'queued'.
+function setBroadcastShow(PDO $db, int $showId): string {
+    $live = liveBroadcast($db);
+    if ($live !== null) {
+        $db->prepare('UPDATE broadcasts SET show_id = ? WHERE id = ?')->execute([$showId, $live['id']]);
+        return 'live';
+    }
+    clearQueuedShow($db);
+    $db->prepare('INSERT INTO broadcasts (show_id) VALUES (?)')->execute([$showId]);
+    return 'queued';
+}
+
+function clearQueuedShow(PDO $db): void {
+    $db->exec('DELETE FROM broadcasts WHERE started_at IS NULL');
+}
+
 // Tracks from the most recent broadcast that has started, newest first
 function recentTracks(PDO $db, int $limit = 100): array {
     $stmt = $db->prepare(