feat(cms): version history panel with diff view and one-click restore

- GET /versions lists the automatic backups of the selected file (timestamp,
  size); ?show=<backup> renders a line diff against the current content
- POST /restore validates the backup against the content schema and restores
  it atomically; the pre-restore state gets a fresh backup first, so a
  restore itself is reversible; audited as version_restored
- dependency-free LCS line diff (scripts/cms-diff.js) with add/del
  highlighting and context trimming; backup names validated against a strict
  pattern (path traversal impossible)
- new 🕘 Verziók entry in the CMS bottom bar
- refactor: security/infra helpers extracted to scripts/cms-core.js to keep
  content-editor.js under the 400-line hard limit
- integration test: list, diff, restore + reversibility backup, traversal
  rejection, CSRF enforcement, auth

Closes MITHOME-64

scripts/cms-versions.js

@@ -0,0 +1,59 @@
+// Version history helpers for the CMS: listing automatic backups from
+// .content-backups, safe backup-name validation and diff assembly.
+const fs = require('fs');
+const path = require('path');
+const { renderDiffHtml } = require('./cms-diff');
+
+// Backup files are named `<fileKey>.<ISO-ish timestamp>.json`
+const BACKUP_NAME_RE = /^(\d{4}-\d{2}-\d{2})T(\d{2})-(\d{2})-(\d{2})-(\d{3})Z$/;
+
+// WHY: the backup name arrives as a query parameter — only allow the exact
+// `<fileKey>.<timestamp>.json` shape so path traversal (`../`) is impossible.
+function safeBackupName(fileKey, candidate) {
+  if (typeof candidate !== 'string' || !candidate.startsWith(`${fileKey}.`) || !candidate.endsWith('.json')) return null;
+  const ts = candidate.slice(fileKey.length + 1, -5);
+  if (!BACKUP_NAME_RE.test(ts)) return null;
+  return candidate;
+}
+
+function formatBackupTimestamp(fileKey, backupName) {
+  const ts = backupName.slice(fileKey.length + 1, -5);
+  const m = ts.match(BACKUP_NAME_RE);
+  if (!m) return ts;
+  return `${m[1]} ${m[2]}:${m[3]}:${m[4]}`;
+}
+
+function listVersions(backupDir, fileKey) {
+  try {
+    return fs.readdirSync(backupDir)
+      .filter(name => safeBackupName(fileKey, name))
+      .map(name => {
+        const full = path.join(backupDir, name);
+        const stat = fs.statSync(full);
+        return { name, size: stat.size, when: formatBackupTimestamp(fileKey, name) };
+      })
+      .sort((a, b) => b.name.localeCompare(a.name)); // newest first
+  } catch {
+    return [];
+  }
+}
+
+function readBackupContent(backupDir, backupName) {
+  return fs.readFileSync(path.join(backupDir, backupName), 'utf8');
+}
+
+// Compare a backup with the current file content; returns both pretty texts and
+// the rendered diff HTML (backup = old/left, current = new/right).
+function buildVersionDiff(backupDir, currentFilePath, fileKey, backupName) {
+  const backupText = readBackupContent(backupDir, backupName);
+  const currentText = fs.readFileSync(currentFilePath, 'utf8');
+  return {
+    backupName,
+    when: formatBackupTimestamp(fileKey, backupName),
+    backupText: backupText.trim(),
+    currentText: currentText.trim(),
+    diffHtml: renderDiffHtml(backupText, currentText),
+  };
+}
+
+module.exports = { safeBackupName, listVersions, readBackupContent, buildVersionDiff, formatBackupTimestamp };