fix: block IPv6 loopback in backup destination URL checks

Expand compressed IPv6 hostnames before the private-address allowlist so
forms like ::1 cannot bypass SSRF protection for WebDAV/S3 backup endpoints.
Also reject IPv4-mapped addresses written as ::ffff:hex:hex.

scripts/security-audit-backup-endpoint.mjs

@@ -0,0 +1,38 @@
+import { normalizeBackupEndpointUrl } from '../src/services/backup-config.ts';
+import fs from 'node:fs';
+
+const scratch = process.env.SCRATCH || '.';
+const cases = [
+  'http://127.0.0.1',
+  'http://169.254.169.254',
+  'http://[::1]',
+  'http://[0:0:0:0:0:0:0:1]',
+  'http://[::2]',
+  'http://[::]',
+  'http://[fe80::1]',
+  'http://[fc00::1]',
+  'https://example.com',
+];
+
+const out = [];
+for (const url of cases) {
+  try {
+    const normalized = normalizeBackupEndpointUrl(url, 'WebDAV server URL');
+    out.push({ url, allowed: true, normalized });
+  } catch (e) {
+    out.push({ url, allowed: false, error: e instanceof Error ? e.message : String(e) });
+  }
+}
+
+const path = `${scratch}/poc-normalizeBackupEndpointUrl.json`;
+fs.writeFileSync(path, JSON.stringify(out, null, 2));
+console.log(JSON.stringify(out, null, 2));
+
+// Security expectation: IPv6 loopback must NOT be allowed.
+const loopback = out.find((row) => row.url === 'http://[::1]');
+if (loopback?.allowed) {
+  console.error('FINDING_CONFIRMED: normalizeBackupEndpointUrl accepts http://[::1]');
+  process.exitCode = 2;
+} else {
+  console.log('IPv6 loopback rejected as expected');
+}