Backfill data missed by devices with a pre-tag-sync token

/api/sync/ only returns rows changed since a device's last sync token.
Any device that synced before tag/time-entry syncing existed already
has a token from that era; pre-existing tags untouched since then were
never "changed since" that token and so were permanently invisible to
that device, even after tag syncing shipped. Adds a sync_format_version
check that ignores the stored token and forces exactly one full resync
per device when it detects an old-format token, backfilling anything
that was missed.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

templates/base.html

@@ -148,7 +148,7 @@
     {% endblock %}
 
     <script src="{% static 'js/offline-db.js' %}?v=7"></script>
-    <script src="{% static 'js/offline-sync.js' %}?v=8"></script>
+    <script src="{% static 'js/offline-sync.js' %}?v=9"></script>
     <script src="{% static 'js/app.js' %}?v=8"></script>
     {% block extra_js %}{% endblock %}
 </body>