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/macros/__init__.py

@@ -82,6 +82,21 @@ def normalize_eurostat_nuts(evaluator, code_col) -> str:
     )
 
 
+@macro()
+def slugify(evaluator, col) -> str:
+    """URL-safe slug: lowercase → ß→ss → strip accents → non-alnum to dashes → trim.
+
+    Usage in SQL: @slugify(city) AS city_slug
+    """
+    c = str(col)
+    return (
+        f"TRIM(REGEXP_REPLACE("
+        f"LOWER(STRIP_ACCENTS(REPLACE(LOWER({c}), 'ß', 'ss'))), "
+        f"'[^a-z0-9]+', '-'"
+        f"), '-')"
+    )
+
+
 @macro()
 def infer_country_from_coords(evaluator, lat_col, lon_col) -> str:
     """Infer ISO country code from lat/lon using bounding boxes for 8 European markets.