feat(transform): individualise article costs with per-country Eurostat data

Add real per-country cost data to ~30 calculator fields so pSEO articles
show country-specific CAPEX/OPEX instead of hardcoded DE defaults.

Extractor:
- eurostat.py: add 8 new datasets (nrg_pc_205, nrg_pc_203, lc_lci_lev,
  5×prc_ppp_ind variants); add optional `dataset_code` field so multiple
  dict entries can share one Eurostat API endpoint

Staging (4 new models):
- stg_electricity_prices — EUR/kWh by country, semi-annual
- stg_gas_prices         — EUR/GJ by country, semi-annual
- stg_labour_costs       — EUR/hour by country, annual (future staffed scenario)
- stg_price_levels       — PLI indices (EU27=100) for 5 categories, annual

Foundation:
- dim_countries (new) — conformed country dimension; eliminates ~50-line CASE
  blocks duplicated in dim_cities/dim_locations; computes ~29 calculator cost
  override columns from PLI ratios and energy price ratios vs DE baseline;
  NULL for DE so calculator falls through to DEFAULTS unchanged
- dim_cities — replace country_name/slug CASE blocks + country_income CTE
  with JOIN dim_countries
- dim_locations — same refactor as dim_cities

Serving:
- pseo_city_costs_de — JOIN dim_countries; add 29 camelCase override columns
  auto-applied by calculator (electricity, heating, rentSqm, hallCostSqm, …)
- planner_defaults — JOIN dim_countries; same 29 cost columns flow through
  to /api/market-data endpoint

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

transform/sqlmesh_padelnomics/CLAUDE.md

@@ -54,6 +54,7 @@ Grain must match reality — use `QUALIFY ROW_NUMBER()` to enforce it.
 
 | Dimension | Grain | Used by |
 |-----------|-------|---------|
+| `foundation.dim_countries` | `country_code` | `dim_cities`, `dim_locations`, `pseo_city_costs_de`, `planner_defaults` — single source for country names, income, PLI/cost overrides |
 | `foundation.dim_venues` | `venue_id` | `dim_cities`, `dim_venue_capacity`, `fct_daily_availability` (via capacity join) |
 | `foundation.dim_cities` | `(country_code, city_slug)` | `serving.city_market_profile` → all pSEO serving models |
 | `foundation.dim_locations` | `(country_code, geoname_id)` | `serving.location_opportunity_profile` — all GeoNames locations (pop ≥1K), incl. zero-court locations |