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/board.php

@@ -80,13 +80,9 @@ if (is_dir(LOG_DIR)) {
 $out['errors'] = array_slice($errors, 0, 20);
 
 // ── Partner reachability ───────────────────────────────────────────────────
-$cacheFile = '/tmp/vv_partner_cache.json';
-$cacheTtl  = 30;
-$partnerData = null;
+$partnerData = vv_cache_read('board_partner', 30);
 
-if (file_exists($cacheFile) && (time() - (int)filemtime($cacheFile)) < $cacheTtl) {
-    $partnerData = json_decode(file_get_contents($cacheFile), true);
-} else {
+if (!$partnerData) {
     // Discover partner hostname dynamically from master.conf (works for any number of hosts)
     $vars    = vv_conf_vars();
     $mine    = vv_get_hostname();
@@ -113,7 +109,7 @@ if (file_exists($cacheFile) && (time() - (int)filemtime($cacheFile)) < $cacheTtl
             'reachable' => $reached,
             'latency'   => $reached ? $elapsed : null,
         ];
-        @file_put_contents($cacheFile, json_encode($partnerData));
+        vv_cache_write('board_partner', $partnerData);
     }
 }
 $out['partner'] = $partnerData;