PHP app layer: consolidate common functions, fix critical bugs, standardize patterns

Consolidations (config.php gains 5 shared utilities):
- vv_format_uptime() replaces 4 inline uptime-formatting blocks
- vv_parse_conf_scalar() replaces vv_arr_scalar/vv_wd_scalar/vv_fb_scalar/vv_media_conf_scalar
- vv_known_hosts() replaces vv_arr_known_hosts/vv_fb_known_hosts + inline parser in watchdog
- vv_parse_kv_db() replaces inline key=value parsing in snapshot and monitor
- vv_local_ip() replaces duplicate in docker_folders.php and inline in docker.php
All module-level function names kept as thin aliases so call sites unchanged.

Critical bug fixes:
- api/system.php: added require_once config.php and POST-only guard (no auth on shutdown)
- api/movescript.php + reorderarray.php: use vv_write_conf_raw (atomic) + vv_push_master_conf
- api/snapshot.php: share /tmp/vv_cpu_stat.json with vv_cpu_per_core() instead of own state file

Correctness:
- vv_cpu_per_core() and vv_network_stats(): atomic tmp+rename for state files (concurrent poll safety)
- ext_ip curl cache moved from /tmp/vv_ext_ip.cache to vv_cache_read/write (canonical cache dir)
- monitor_remote.php + board.php + snapshot.php: all use vv_cache_read/write instead of ad-hoc /tmp files

HTTP method guards added to write-only APIs that were missing them:
- api/scheduler.php, conf_toggle.php, flag_toggle.php

Plugin/unraid/api/monitor_remote.php

@@ -3,18 +3,11 @@ header('Content-Type: application/json');
 header('Cache-Control: no-cache, no-store');
 require_once dirname(__DIR__) . '/include/monitor.php';
 
-$cacheFile = '/tmp/vv_cache_monitor_remote.json';
-$cacheTTL  = 3600;
-
-if (!isset($_GET['live']) && file_exists($cacheFile) && (time() - filemtime($cacheFile)) < $cacheTTL) {
-    echo file_get_contents($cacheFile);
-    exit;
+if (!isset($_GET['live'])) {
+    $cached = vv_cache_read('monitor_remote', 3600);
+    if ($cached) { echo json_encode($cached); exit; }
 }
 
-$out = json_encode([
-    'remote_hosts' => vv_remote_hosts_stats(),
-    'ts'           => time(),
-]);
-
-file_put_contents($cacheFile, $out);
-echo $out;
+$data = ['remote_hosts' => vv_remote_hosts_stats(), 'ts' => time()];
+vv_cache_write('monitor_remote', $data);
+echo json_encode($data);