feat: server monitoring charts, package limits, WHMCS bridge, server options (#19-22)

#19 Server monitoring charts:
- server_stats table (migration 007) + collect-stats.php cron script
- serverStatus() page rebuilt with Chart.js line charts (CPU/RAM/disk)
- Chart.js lazy-loaded from CDN; history shown for last 24h

#20 Cron job manager: already complete in prior session

#21 Package limits enforcement:
- email.php: checks max_email before creating email account
- databases.php: checks max_databases before creating database
- ftp.php: checks max_ftp before creating FTP account
- stats.php: fixed column names (max_email/max_ftp/max_databases vs old aliases)

#22b WHMCS billing bridge:
- whmcs.php endpoint: create/suspend/unsuspend/terminate/changepackage/info
- Auth via X-WHMCS-Key header; enabled/key stored in settings table

#22a/c/d/e Server options admin page:
- Web/mail/FTP/DNS server selection with settings persistence
- Server switch triggers /opt/novacpx/bin/switch-*.sh scripts (if present)
- NS health checker: live dig lookup of all zones vs configured NS1/NS2
- system.php: server-options + save-option actions
- dns.php: ns-health action

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

panel/api/endpoints/stats.php

@@ -76,17 +76,17 @@ match ($action) {
         $pkg = $db->fetchOne("SELECT * FROM packages WHERE id = ?", [$acct['package_id'] ?? 0]);
 
         Response::success([
-            'disk_mb'     => $diskMB,
-            'disk_limit'  => $pkg['disk_mb'] ?? 0,
-            'inodes'      => $inodes,
-            'databases'   => $dbCount,
-            'db_limit'    => $pkg['databases'] ?? 0,
-            'emails'      => $emailCount,
-            'email_limit' => $pkg['email_accounts'] ?? 0,
-            'ftp'         => $ftpCount,
-            'ftp_limit'   => $pkg['ftp_accounts'] ?? 0,
-            'domains'     => $domCount,
-            'subdomain_limit' => $pkg['subdomains'] ?? 0,
+            'disk_mb'         => $diskMB,
+            'disk_limit'      => $pkg['disk_mb']         ?? 0,
+            'inodes'          => $inodes,
+            'databases'       => $dbCount,
+            'db_limit'        => $pkg['max_databases']   ?? 0,
+            'emails'          => $emailCount,
+            'email_limit'     => $pkg['max_email']       ?? 0,
+            'ftp'             => $ftpCount,
+            'ftp_limit'       => $pkg['max_ftp']         ?? 0,
+            'domains'         => $domCount,
+            'subdomain_limit' => $pkg['max_subdomains']  ?? 0,
         ]);
     })(),