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/services/scheduler_service.py

@@ -15,7 +15,7 @@ def init_scheduler():
     logger.info("Scheduler initialized")
 
 
-def start_scheduler(interval_minutes: int = 5):
+def start_scheduler(interval_minutes: int = 10):
     """Start the scheduler with the given interval."""
     global _is_running
     if _scheduler is None: