fix(content): slugify transliteration + article links + country overview ranking

- Add @slugify SQLMesh macro (STRIP_ACCENTS + ß→ss) replacing broken
  inline REGEXP_REPLACE that dropped non-ASCII chars (Düsseldorf → d-sseldorf)
- Apply @slugify to dim_venues, dim_cities, dim_locations
- Fix Python slugify() to pre-replace ß→ss before NFKD normalization
- Add language prefix to B2B article market links (/markets/germany → /de/markets/germany)
- Change country overview top-5 ranking: venue count (not raw market_score)
  for top cities, population for top opportunity cities

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

transform/sqlmesh_padelnomics/models/foundation/dim_venues.sql

@@ -99,7 +99,7 @@ SELECT
   indoor_court_count,
   outdoor_court_count,
   -- Conformed city key: enables deterministic joins to dim_cities / venue_pricing_benchmarks
-  LOWER(REGEXP_REPLACE(LOWER(COALESCE(city, '')), '[^a-z0-9]+', '-')) AS city_slug,
+  @slugify(COALESCE(city, '')) AS city_slug,
   extracted_date
 FROM ranked
 QUALIFY ROW_NUMBER() OVER (