map: fixed Berlin view, CartoDB Voyager tiles, OpenRailwayMap transit overlay

- Drop the fitBounds-to-markers logic so the map always opens at the same
  Berlin-wide view (center 52.52/13.405, zoom 11), regardless of pin count
- Swap OSM standard tiles for CartoDB Voyager — cleaner, more
  Google-Maps-like base style
- Add OpenRailwayMap overlay (opacity .75) so S-/U-Bahn/Tram lines are
  highlighted on top of the base
- CSP img-src widened to cover the new tile hosts

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

web/app.py

@@ -134,7 +134,11 @@ async def security_headers(request: Request, call_next):
         "default-src 'self'; "
         "script-src 'self' https://cdn.tailwindcss.com https://unpkg.com; "
         "style-src 'self' https://cdn.tailwindcss.com https://unpkg.com 'unsafe-inline'; "
-        "img-src 'self' data: https://*.tile.openstreetmap.org https://tile.openstreetmap.org https://unpkg.com; "
+        "img-src 'self' data: "
+        "https://*.tile.openstreetmap.org https://tile.openstreetmap.org "
+        "https://*.basemaps.cartocdn.com https://basemaps.cartocdn.com "
+        "https://*.tiles.openrailwaymap.org https://tiles.openrailwaymap.org "
+        "https://unpkg.com; "
         "connect-src 'self'; frame-ancestors 'none';"
     )
     return resp