Map tiles: switch to Esri World Light Gray (OSM 403s file://)

tiles="OpenStreetMap" returned HTTP 403 "referer is required by tile usage
policy" in the target browser — OSM graylists refererless requests and a
karte.html opened via file:// sends no Referer. Esri's Canvas/World_Light_Gray
base + reference layers need neither a key nor a Referer and keep the muted
look. New MAP_TILES / MAP_TILES_LABELS / MAP_TILES_ATTR constants; generate_map
adds them as explicit TileLayers.

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

docs/overview.md

@@ -65,9 +65,10 @@ A table view of everything in `orte.csv`.
 
 Generated by `generate_map()`:
 
-- 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.
+- Base layer: **Esri "World Light Gray"** (`MAP_TILES` + a labels overlay) – a
+  light, muted basemap that needs no API key and no `Referer`. Initial view
+  centred on `[49.0, 9.0]`, zoom 8; 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`).
@@ -77,11 +78,12 @@ Generated by `generate_map()`:
   bar shows a warning).
 
 > 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).
+> tiles from `server.arcgisonline.com`, so the map needs an internet connection
+> to render even though the data is local. To change the look, edit `MAP_TILES` /
+> `MAP_TILES_LABELS` near the top of `app.py` — the comment there lists other
+> no-key Esri layers (Street, Topo). `tile.openstreetmap.org` was tried first
+> but its tile-usage policy **403s** pages opened from `file://` (no `Referer`);
+> `CartoDB positron` needs an API key.
 
 ## Geocoding behaviour