Add DKIM auto-provisioning, OS/panel self-update with self-healing

- AccountManager: auto-generate DKIM keypair + inject SPF/DKIM/DMARC DNS records on account create
- AccountManager: rotateDKIM() method for key rotation with new selector
- New dkim.php endpoint: list/view/rotate/provision DKIM keys per domain
- schema.sql: add dkim_keys table
- install.sh: install opendkim, wire into Postfix milter, fix dotfile copy (. vs *), fix config.ini permissions (root:www-data 640), copy VERSION to web root, add opendkim to service restart
- api/index.php: fix NOVACPX_ROOT path (was 2 levels too high), fix CORS ports (8880-8883), VERSION fallback to /opt/novacpx-src
- api/.htaccess: route all /api/* requests through index.php
- system.php: check-os-update, apply-os-update (self-healing: auto-restart downed services, restore web root if panel ports go down), check-novacpx-update, apply-novacpx-update (PHP syntax validation before deploy, backup + restore on failure)
- admin.js: Updates page now shows both NovaCPX panel updates and OS package upgrades in one section; sidebar badge shows combined count

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

panel/public/api/.htaccess

@@ -0,0 +1,4 @@
+Options -Indexes
+RewriteEngine On
+RewriteCond %{REQUEST_FILENAME} !-f
+RewriteRule ^(.*)$ index.php [QSA,L]