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/main.py

@@ -53,7 +53,7 @@ async def lifespan(app: FastAPI):
     try:
         config = await sqlite_service.get_scheduler_config()
         if config.get("enabled") == "True":
-            interval = int(config.get("interval_minutes", "5"))
+            interval = int(config.get("interval_minutes", "10"))
             scheduler_service.start_scheduler(interval)
     except Exception:
         pass