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_locations.sql

@@ -38,7 +38,7 @@ locations AS (
     geoname_id,
     city_name                                                            AS location_name,
     -- URL-safe location slug
-    LOWER(REGEXP_REPLACE(LOWER(city_name), '[^a-z0-9]+', '-'))          AS location_slug,
+    @slugify(city_name)                                                 AS location_slug,
     country_code,
     lat,
     lon,