N+1 query in sync_vendor_data - batch data_source lookups and event_record inserts

`sync_vendor_data_task` executes the same `SELECT … FROM data_source` query once per record during sync, resulting in an N+1 problem. In a single transaction the query pattern repeats **~85 times** alongside individual `INSERT INTO event_record` and `INSERT INTO event_record_detail` statements. **Impact:** 255 occurrences detected on our internal instance since Feb 6. Adds unnecessary DB load on every vendor data sync. **Fix:** Pre-fetch/cache `data_source` lookups before the loop and batch `event_record` / `event_record_detail` inserts using `bulk_save_objects` or `executemany`.