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>

app.py

@@ -58,6 +58,10 @@ GEOCODE_REGIONS = ["Baden-Württemberg", "Hessen"]
 MAP_DEFAULT_CENTER = [49.0, 9.0]
 MAP_DEFAULT_ZOOM = 8
 
+#: Kartenkacheln. "OpenStreetMap" braucht keinen API-Schlüssel.
+#: (Die früher genutzten "CartoDB positron"-Kacheln verlangen seit 2024 einen Schlüssel.)
+MAP_TILES = "OpenStreetMap"
+
 log = logging.getLogger("arbeitsorte")
 
 
@@ -415,7 +419,7 @@ def generate_map(store: DataStore) -> Path:
 
     m = folium.Map(
         location=MAP_DEFAULT_CENTER, zoom_start=MAP_DEFAULT_ZOOM,
-        tiles="CartoDB positron",
+        tiles=MAP_TILES,
     )
     max_count = int(freq["count"].max())