Fix roofz parser for new JSON API

[wtfloris]

Summary

• Roofz migrated from Nuxt SSR (/availability) to a Marketsuite JSON API (/api/ms/listing/properties)
• Rewrote parser to consume the new paginated JSON API instead of scraping __NUXT__ inline data
• Now extracts living_area (sqm) from the API response
• Updated listing URLs from /availability/{slug} to /huur/woningen/{slug}
• Removed _substitute_nuxt_vars helper (no longer needed)

Test plan

• All 92 parser tests pass
• Verified live API returns data and parser produces correct results
• Verified at least 3 parsed listing URLs return HTTP 200
• End-to-end test in Docker: scraper inserts listings with correct address, city, price, and sqm

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