fix(scheduler): prevent auto-enable with wrong interval on page load

Root cause: GET /api/sync/schedule returned interval_minutes=null when
scheduler was stopped, causing dropdown to stay on first HTML option
(1 min). Setting .value programmatically could trigger onchange, sending
a second PUT with interval=1 right after the user's intended interval.

- GET schedule falls back to DB/default (10 min) when scheduler is off
- Add _schedulerLoading flag to block onchange during loadSchedulerStatus
- Default interval 10 min everywhere (was 5 in backend)
- Cache bust dashboard.js v=33

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

api/app/templates/dashboard.html

@@ -114,5 +114,5 @@
 {% endblock %}
 
 {% block scripts %}
-<script src="{{ request.scope.get('root_path', '') }}/static/js/dashboard.js?v=32"></script>
+<script src="{{ request.scope.get('root_path', '') }}/static/js/dashboard.js?v=33"></script>
 {% endblock %}