docs: update CHANGELOG with H3 catchment score v3

CHANGELOG.md

@@ -6,6 +6,9 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/).
 
 ## [Unreleased]
 
+### Changed
+- **Marktpotenzial-Score v3: H3 catchment lens** — addressable market (25pts) and supply gap (30pts) now use a regional H3 catchment (~15-18km radius, res-4 cell + 6 neighbours, ~462km²) instead of local city population and 5km court count. Mid-size cities surrounded by dense Gemeinden (e.g. Oldenburg) now score correctly. New output columns: `catchment_population`, `catchment_padel_courts`, `catchment_venues_per_100k`. Requires one-time `INSTALL h3 FROM community` in DuckDB on each machine.
+
 ### Added
 - **Custom 404/500 error pages** — styled error pages extending `base.html` with i18n support (EN/DE). The 404 page is context-aware: when the URL matches `/markets/{country}/{city}`, it shows a city-specific message with a link back to the country overview instead of a generic "page not found".
 - **Map: city article indicators** — country overview map bubbles now differentiate cities with/without published articles. All cities retain score-based colors (green/amber/red); non-article cities are visually receded with lower opacity, dashed borders, desaturated color, and default cursor (no click). Tooltips show scores for all cities — article cities get "Click to explore →", non-article cities get "Coming soon". The `/api/markets/<country>/cities.json` endpoint includes a `has_article` boolean per city.