Add the central site registry and remote updates

Every installation now reports address and version to sites.kant-it.de
every 15 minutes and picks up update orders there (ADR 123). Reports and
answers are HMAC-signed with a per-installation key sent on first
contact; an order can only install a published release with checksum.
The next request after the file swap runs the migrations, so a remote
update finishes without anyone in the admin.

The platform itself lives in zentrale/ (plain PHP + SQLite, not part of
the release archive): overview with outdated, silent and unassigned
installations, single and bulk updates, per-site detail and history.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

scripts/build-release.php

@@ -68,7 +68,7 @@ echo "Fassung {$version}\n";
 // Einrichten keine Migrationsliste und bricht mit "configuration file does not exist" ab - genau
 // dort, wo eine frische Installation ihre Tabellen anlegen will.
 $include = ['bin', 'config', 'migrations', 'plugins', 'public', 'src', 'templates', 'composer.json', 'composer.lock', 'migrations.php', 'cli-config.php', 'VERSION', 'README.md', '.env.example', '.htaccess'];
-$skip = ['.git', '.github', '.claude', '.gitattributes', '.gitignore', '.env.lock', '.php-cs-fixer.cache', 'node_modules', 'var', 'tests', 'scripts', 'reports', 'phpunit.xml.dist', '.php-cs-fixer.dist.php', 'package.json', 'package-lock.json', 'AGENTS.md', 'ENTWICKLUNGSPLAN.md', 'docs', 'composer.phar', '.venv', 'vendor', 'phpunit.cache', 'migrations.php.bak'];
+$skip = ['.git', '.github', '.claude', '.gitattributes', '.gitignore', '.env.lock', '.php-cs-fixer.cache', 'node_modules', 'var', 'tests', 'scripts', 'reports', 'phpunit.xml.dist', '.php-cs-fixer.dist.php', 'package.json', 'package-lock.json', 'AGENTS.md', 'ENTWICKLUNGSPLAN.md', 'docs', 'composer.phar', '.venv', 'vendor', 'phpunit.cache', 'migrations.php.bak', 'zentrale'];
 
 $copy = static function (string $from, string $to) use (&$copy, $skip): int {
     $count = 0;