Switch map tiles from CartoDB positron to OpenStreetMap (no API key)

CartoDB's positron basemap now renders an "API KEY REQUIRED" watermark.
Use tiles="OpenStreetMap" via a new MAP_TILES constant instead.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

docs/dev-notes.md

@@ -36,6 +36,20 @@ Numeric parsing is deliberately deferred to `get_map_data` (`pd.to_numeric(...,
 errors="coerce")`). Don't "fix" columns to float on load — blank coordinates and
 leading-zero PLZ both depend on the string representation.
 
+## CartoDB "positron" tiles now require an API key (2026-09-07)
+
+The original `folium.Map(tiles="CartoDB positron")` started rendering an
+"API KEY REQUIRED / carto.com/basemaps/apikey" watermark – Carto moved its
+basemaps behind a key. Switched to `tiles="OpenStreetMap"` (constant
+`MAP_TILES`), which needs no key. Trade-off: OSM standard tiles are more
+colourful than positron's muted grey. Other no-key options if a lighter look is
+wanted: OSM Germany (`tile.openstreetmap.de`), OSM France Hot
+(`{s}.tile.openstreetmap.fr/hot`) – both still colourful. Truly muted
+key-free basemaps are hard to find now.
+
+Don't assume a "free" third-party tile source stays free – pin the provider in
+one constant so swapping it is a one-line change.
+
 ## Nominatim query order matters
 
 `geocode_city` returns the **first** hit, trying PLZ-qualified first, then each