The get_ids_to_process function was using set union instead of set
difference, causing it to return all existing listing IDs along with
new ones. This meant:
1. When there were no new listings, the task would iterate through all
existing listings, find nothing to process for each, and complete
almost instantly
2. The task showed no progress because processing was too fast
Fixed by:
- Changed `all_listing_ids.union(identifiers)` to `identifiers - all_listing_ids`
to only return IDs that are NOT already in the database
- Added explicit check for empty set with informative task state
"No new listings found" so users understand why the task completed quickly
