Replace the sequential fetch-all-then-process pipeline with a streaming
architecture where listing processing starts as soon as IDs become
available from each subquery. A producer task fetches pages and enqueues
new IDs (filtered inline against DB), while 20 consumer workers process
listings concurrently from the queue.
- Add ListingRepository.get_listing_ids() for fast ID-only projection
- Refactor listing_tasks.py: remove get_ids_to_process/dump_listings_and_monitor,
replace with unified producer/worker/monitor pipeline
- Apply same pattern to CLI path in listing_fetcher.py
- Remove 'filtering' phase from frontend, show combined fetch+process metrics
- Add fetching_done flag to TaskResult for phase transition tracking
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