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>

app.py

@@ -61,9 +61,16 @@ 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"
+#: Kartenkacheln – Esri "World Light Gray" (heller, ruhiger Hintergrund).
+#: Braucht keinen API-Schlüssel und keinen Referer (anders als tile.openstreetmap.org,
+#: das lokale Dateien mit 403 blockt, und "CartoDB positron", das einen Schlüssel will).
+#: Alternativen ohne Schlüssel als Ein-Zeilen-Tausch:
+#:   Esri Straßenkarte:  …/World_Street_Map/MapServer/tile/{z}/{y}/{x}
+#:   Esri Topo:          …/World_Topo_Map/MapServer/tile/{z}/{y}/{x}
+_ESRI = "https://server.arcgisonline.com/ArcGIS/rest/services"
+MAP_TILES = f"{_ESRI}/Canvas/World_Light_Gray_Base/MapServer/tile/{{z}}/{{y}}/{{x}}"
+MAP_TILES_LABELS = f"{_ESRI}/Canvas/World_Light_Gray_Reference/MapServer/tile/{{z}}/{{y}}/{{x}}"
+MAP_TILES_ATTR = "Kacheln © Esri"
 
 log = logging.getLogger("arbeitsorte")
 
@@ -421,9 +428,15 @@ def generate_map(store: DataStore) -> Path:
         raise ValueError("Keine Koordinaten vorhanden. Bitte zuerst Einträge speichern.")
 
     m = folium.Map(
-        location=MAP_DEFAULT_CENTER, zoom_start=MAP_DEFAULT_ZOOM,
-        tiles=MAP_TILES,
+        location=MAP_DEFAULT_CENTER, zoom_start=MAP_DEFAULT_ZOOM, tiles=None,
     )
+    folium.TileLayer(
+        MAP_TILES, attr=MAP_TILES_ATTR, name="Karte", control=False,
+    ).add_to(m)
+    folium.TileLayer(
+        MAP_TILES_LABELS, attr=MAP_TILES_ATTR, name="Beschriftung",
+        control=False, overlay=True,
+    ).add_to(m)
     max_count = int(freq["count"].max())
 
     for _, row in freq.iterrows():