Add Yunohost setup guide and make the poller lock per-script

The poller's lock file in /tmp could be left owned by another user after a
manual run, leaving cron unable to open it. Lock the script file instead.

SETUP.md walks through installing the site on Yunohost, including the
/etc/cron.d entry for the poller, which was missing on the server and left
the recently played list empty.

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

cli/poll_icecast.php

@@ -11,8 +11,13 @@ if (PHP_SAPI !== 'cli') {
 require __DIR__ . '/../includes/db.php';
 require __DIR__ . '/../includes/broadcasts.php';
 
-// Skip this run if the previous one is still going
-$lock = fopen(sys_get_temp_dir() . '/rfe_poll_icecast.lock', 'c');
+// Skip this run if the previous one is still going. Locks this script rather than a file in /tmp,
+// which another user (e.g. a manual run over SSH) could own and leave unopenable for cron.
+$lock = fopen(__FILE__, 'r');
+if ($lock === false) {
+    fwrite(STDERR, date('c') . " Unable to open lock\n");
+    exit(1);
+}
 if (!flock($lock, LOCK_EX | LOCK_NB)) exit;
 
 for ($i = 0; $i < 4; $i++) {