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>

CLAUDE.md

@@ -43,7 +43,7 @@ MariaDB on localhost. Credentials live in `config/db_credentials.php`, which is
 - `broadcasts` — one row per streaming session: pending (queued by `cli/set_show.php`), live, or ended.
 - `played_tracks` — tracks per broadcast. Cleared ten minutes after the broadcast ends; no long-term history is kept.
 
-`cli/poll_icecast.php` runs from cron every minute and polls Icecast four times, 15 seconds apart. A broadcast starts when the stream comes online and ends after five minutes unseen (so brief drop-outs don't split a show). Before going live, run `php cli/set_show.php <slug>` to choose the show.
+`cli/poll_icecast.php` runs from cron every minute and polls Icecast four times, 15 seconds apart. A broadcast starts when the stream comes online and ends after five minutes unseen (so brief drop-outs don't split a show). Before going live, choose the show on the admin dashboard (or run `php cli/set_show.php <slug>`). Shows and their background images are managed at `admin/shows.php`; uploads are saved to `images/backgrounds/` under generated names, so the web server needs write access to that directory.
 
 ## Admin Authentication