feat: embed git hash in website and verify after CI deploy

- website/layouts/_partials/extend_head.html: injects <meta name="x-version">
  using HUGO_PARAMS_GITVERSION (set by Taskfile at build time)
- Taskfile: website-build sets HUGO_PARAMS_GITVERSION=<short HEAD>;
  new website-verify task runs scripts/website-verify.sh
- scripts/website-verify.sh: fetches homepage, retries 6x/10s, checks
  that the deployed version hash matches HEAD
- website.yml: Verify step after Deploy; scripts/website-verify.sh added
  to path trigger

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

scripts/website-verify.sh

@@ -0,0 +1,18 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+VERSION=$(git rev-parse --short HEAD)
+URL="https://sharedinbox.de/"
+
+echo "Checking that version ${VERSION} is live at ${URL} ..."
+for i in $(seq 1 6); do
+    if curl -sf "${URL}" | grep -q "x-version.*${VERSION}"; then
+        echo "OK: version ${VERSION} is live."
+        exit 0
+    fi
+    echo "Not yet (attempt ${i}/6); waiting 10s ..."
+    sleep 10
+done
+
+echo "FAIL: version ${VERSION} not found at ${URL}"
+exit 1