Pin Booking.com search to a dest_id

Free-text ss= destination resolution is non-deterministic: tonight's
30-day run resolved "Stow on the Wold" to St. Wolfgang, Austria for 8 of
30 dates, saving Salzkammergut hotel rates into the matrix. dest_id +
dest_type=city in the search URL pins the destination.

- booking_scrape_config gains a dest_id column (idempotent ALTER)
- scrape_location_search/_build_search_url thread dest_id through
- /config/location accepts dest_id

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

backend/services/scraper_backends/base.py

@@ -90,7 +90,8 @@ class ScraperBackend(ABC):
         check_in: date,
         check_out: date,
         adults: int = 2,
-        pages: int = 2
+        pages: int = 2,
+        dest_id: Optional[str] = None
     ) -> ScraperResult:
         """
         Scrape booking.com location search results.
@@ -101,6 +102,9 @@ class ScraperBackend(ABC):
             check_out: Check-out date (typically check_in + 1 for single night)
             adults: Number of adults for search
             pages: Number of search result pages to scrape
+            dest_id: Booking.com numeric destination id. Pins the search to one
+                destination — free-text ss= resolves non-deterministically
+                (Stow on the Wold intermittently matched St. Wolfgang, Austria)
 
         Returns:
             ScraperResult with hotels and rates found