add scrapers: Van Daal (API), Van Silfhout (SSR) for Delft

- fetch_vandaal: OG Online API, covers Delft/Rijswijk/Den Haag area,
  includes is_bought→verkocht status mapping
- fetch_vansilfhout: HTML scraper, all listings on single page,
  extracts postcode from embedded JS variable (objectZipcode)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/test_adapters.py

@@ -16,7 +16,7 @@ logging.basicConfig(
 )
 
 # --- change this to test a different adapter ---
-ADAPTER = SCRAPERS['schielandborsboom']
+ADAPTER = SCRAPERS['vansilfhout']
 
 if __name__ == "__main__":
     print(f"Testing adapter: {ADAPTER.__name__}")