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/conf_toggle.php

@@ -2,6 +2,11 @@
 header('Content-Type: application/json');
 require_once dirname(__DIR__) . '/include/scheduler.php';
 
+if ($_SERVER['REQUEST_METHOD'] !== 'POST') {
+    echo json_encode(['ok' => false, 'error' => 'POST only']);
+    exit;
+}
+
 $id      = trim($_POST['id']      ?? '');
 $enabled = ($_POST['enabled'] ?? '0') === '1';