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/overview.md

@@ -62,8 +62,9 @@ A table view of everything in `orte.csv`.
 
 Generated by `generate_map()`:
 
-- Base layer: `CartoDB positron`, initial view centred on `[49.0, 9.0]`,
-  zoom 8 (roughly Baden-Württemberg).
+- Base layer: **OpenStreetMap** standard tiles (`MAP_TILES`) – no API key.
+  Initial view centred on `[49.0, 9.0]`, zoom 8 (roughly Baden-Württemberg);
+  the view auto-fits to the markers when there is more than one.
 - One **`CircleMarker`** per unique `(city, lat, lon)` group. Radius scales
   linearly with the visit count for that location
   (`5 + count / max_count * 20` px), colour is DRK red (`#CC0000`).
@@ -72,8 +73,12 @@ Generated by `generate_map()`:
 - If there are no usable coordinates at all, a dialog says so (and the status
   bar shows a warning).
 
-> The generated HTML pulls Leaflet, jQuery and Bootstrap from CDNs, so the map
-> needs an internet connection to render even though the data is local.
+> The generated HTML pulls Leaflet, jQuery and Bootstrap from CDNs, and the map
+> tiles from `tile.openstreetmap.org`, so the map needs an internet connection to
+> render even though the data is local. To change the look, set `MAP_TILES` near
+> the top of `app.py` to another no-key provider Folium knows
+> (`"OpenStreetMap"`, `"CartoDB positron"` *now needs a key*, or a custom
+> `folium.TileLayer` URL such as OSM Germany / OSM France).
 
 ## Geocoding behaviour