Implement exhibitor search constraints and sync telemetry

- Restricted public exhibitor search to Priority (paid) items only.
- Enforced 3-character search minimum for non-trusted users on landing page.
- Implemented real-time sync telemetry (last refresh and countdown) in Manage tab.
- Added auto-refresh logic with configurable intervals.

src/lib/stores/ae_events_stores.ts

@@ -477,6 +477,9 @@ const events_session_data_struct: key_val = {
 
         submit_status__search: null, // 'searching', 'complete'
 
+        last_refresh_time: null as string | null,
+        next_refresh_countdown: 0,
+
         // The entered_passcode is the exhibit booths shared passcode for staff. This is used to initially access the lead retrieval service.
         entered_passcode: null,