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the release archive): overview with outdated, silent and unassigned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>

zentrale/app/Ansicht.php

@@ -0,0 +1,57 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Zentrale;
+
+/**
+ * Vorlagen als PHP-Dateien. Alles, was ausgegeben wird, geht durch e().
+ */
+final class Ansicht
+{
+    public function __construct(private string $csrf)
+    {
+    }
+
+    /** @param array<string, mixed> $daten */
+    public function vorlage(string $datei, array $daten): string
+    {
+        $v = $this;
+        extract($daten, EXTR_SKIP);
+        ob_start();
+        require $datei;
+
+        return (string) ob_get_clean();
+    }
+
+    public function e(mixed $wert): string
+    {
+        return htmlspecialchars((string) $wert, ENT_QUOTES | ENT_SUBSTITUTE, 'UTF-8');
+    }
+
+    public function csrf(): string
+    {
+        return '<input type="hidden" name="_csrf" value="'.$this->e($this->csrf).'">';
+    }
+
+    /** Ein Zeitpunkt in deutscher Ortszeit. */
+    public function zeit(?string $iso): string
+    {
+        if (null === $iso || '' === $iso || false === strtotime($iso)) {
+            return '–';
+        }
+
+        return (new \DateTimeImmutable($iso))->setTimezone(new \DateTimeZone('Europe/Berlin'))->format('d.m.Y, H:i');
+    }
+
+    /** „vor 5 Minuten“ - für die letzte Meldung. */
+    public function vor(int $sekunden): string
+    {
+        return match (true) {
+            $sekunden < 90 => 'gerade eben',
+            $sekunden < 5400 => sprintf('vor %d Minuten', (int) round($sekunden / 60)),
+            $sekunden < 129600 => sprintf('vor %d Stunden', (int) round($sekunden / 3600)),
+            default => sprintf('vor %d Tagen', (int) round($sekunden / 86400)),
+        };
+    }
+}