Stream-process listings as IDs arrive via asyncio.Queue

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

crawler/repositories/listing_repository.py

@@ -343,6 +343,20 @@ class ListingRepository:
 
         return model_listing
 
+    def get_listing_ids(
+        self,
+        listing_type: ListingType = ListingType.RENT,
+    ) -> set[int]:
+        """Get all listing IDs from the database (ID-only projection).
+
+        Much faster than get_listings() when only IDs are needed for
+        filtering against API results.
+        """
+        model = RentListing if listing_type == ListingType.RENT else BuyListing
+        with Session(self.engine) as session:
+            result = session.execute(sa_select(model.id))
+            return {row[0] for row in result.fetchall()}
+
     async def mark_seen(self, listing_id: int) -> None:
         listings = await self.get_listings(only_ids=[listing_id])
         if len(listings) == 0: