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/static/js/dashboard.js

@@ -11,6 +11,7 @@ let _lastRunId = null;
 let _currentRunId = null;
 let _pollIntervalMs = 5000; // default, overridden from settings
 let _knownLastRunId = null; // track last_run.run_id to detect missed syncs
+let _schedulerLoading = false; // prevent onchange during programmatic load
 
 // ── Init ──────────────────────────────────────────
 
@@ -202,6 +203,7 @@ async function toggleScheduler() {
 }
 
 async function updateSchedulerInterval() {
+    if (_schedulerLoading) return; // ignore programmatic changes during load
     const enabled = document.getElementById('schedulerToggle').checked;
     if (enabled) {
         await toggleScheduler();
@@ -209,15 +211,16 @@ async function updateSchedulerInterval() {
 }
 
 async function loadSchedulerStatus() {
+    _schedulerLoading = true;
     try {
         const res = await fetch('/api/sync/schedule');
         const data = await res.json();
         document.getElementById('schedulerToggle').checked = data.enabled || false;
-        if (data.interval_minutes) {
-            document.getElementById('schedulerInterval').value = data.interval_minutes;
-        }
+        document.getElementById('schedulerInterval').value = data.interval_minutes || 10;
     } catch (err) {
         console.error('loadSchedulerStatus error:', err);
+    } finally {
+        _schedulerLoading = false;
     }
 }