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>

db/migrations/007_stats_settings.sql

@@ -0,0 +1,22 @@
+-- Migration 007: server_stats table + server type settings + WHMCS key
+CREATE TABLE IF NOT EXISTS server_stats (
+    id          INT UNSIGNED AUTO_INCREMENT PRIMARY KEY,
+    cpu_usage   DECIMAL(5,2)  DEFAULT 0,
+    ram_usage   DECIMAL(5,2)  DEFAULT 0,
+    disk_usage  DECIMAL(5,2)  DEFAULT 0,
+    load_avg    DECIMAL(8,2)  DEFAULT 0,
+    net_in_kb   BIGINT        DEFAULT 0,
+    net_out_kb  BIGINT        DEFAULT 0,
+    recorded_at TIMESTAMP     DEFAULT CURRENT_TIMESTAMP,
+    INDEX (recorded_at)
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;
+
+INSERT IGNORE INTO settings (`key`, `value`) VALUES
+    ('web_server',   'apache'),
+    ('mail_server',  'postfix-dovecot'),
+    ('ftp_server',   'proftpd'),
+    ('dns_server',   'bind9'),
+    ('whmcs_api_key', ''),
+    ('whmcs_enabled', '0'),
+    ('ns1_hostname', ''),
+    ('ns2_hostname', '');