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/booking_scraper.py

@@ -82,7 +82,7 @@ def get_scrape_config(db: Session) -> Optional[Dict[str, Any]]:
     """Get the active scrape location configuration."""
     result = db.execute(
         text("""
-            SELECT id, location_name, location_search_url, pages_to_scrape, adults
+            SELECT id, location_name, location_search_url, pages_to_scrape, adults, dest_id
             FROM booking_scrape_config
             WHERE is_active = TRUE
             ORDER BY id
@@ -99,6 +99,7 @@ def get_scrape_config(db: Session) -> Optional[Dict[str, Any]]:
         'location_search_url': result.location_search_url,
         'pages_to_scrape': result.pages_to_scrape or 2,
         'adults': result.adults or 2,
+        'dest_id': result.dest_id,
     }
 
 
@@ -330,7 +331,8 @@ async def scrape_date(
         check_in=check_in,
         check_out=check_out,
         adults=config['adults'],
-        pages=config['pages_to_scrape']
+        pages=config['pages_to_scrape'],
+        dest_id=config.get('dest_id'),
     )
 
     if result.blocked: