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/email.php

@@ -30,6 +30,12 @@ match ($action) {
         if (!filter_var($email, FILTER_VALIDATE_EMAIL)) Response::error("Invalid email address");
         if (strlen($password) < 6) Response::error("Password must be at least 6 characters");
         if (!$accountId) Response::error("account_id required");
+        // Package limit check
+        $acctPkg = $db->fetchOne("SELECT p.max_email FROM accounts a LEFT JOIN packages p ON p.id=a.package_id WHERE a.id=?", [$accountId]);
+        if ($acctPkg && $acctPkg['max_email'] > 0) {
+            $count = (int)$db->fetchOne("SELECT COUNT(*) c FROM email_accounts WHERE account_id=?", [$accountId])['c'];
+            if ($count >= (int)$acctPkg['max_email']) Response::error("Email account limit ({$acctPkg['max_email']}) reached for this package", 403);
+        }
         $id = EmailManager::createAccount($accountId, $email, $password, $quota);
         audit('email.create', $email);
         Response::success(['id' => $id], "Email account created: $email");