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/static/map.js

@@ -48,7 +48,6 @@ function renderMarkers(data) {
     markerLayer = L.layerGroup().addTo(mapInstance);
   }
 
-  const bounds = [];
   data.forEach((f) => {
     if (typeof f.lat !== "number" || typeof f.lng !== "number") return;
     const rent = f.rent ? Math.round(f.rent) + " €" : "";
@@ -61,14 +60,7 @@ function renderMarkers(data) {
       `<b>${safeAddr}</b>` + (meta ? `<br>${meta}` : "") +
       `<br><a href="${safeLink}" target="_blank" rel="noopener">Zur Anzeige →</a>`,
     );
-    bounds.push([f.lat, f.lng]);
   });
-
-  if (bounds.length === 1) {
-    mapInstance.setView(bounds[0], 14);
-  } else if (bounds.length > 1) {
-    mapInstance.fitBounds(bounds, { padding: [30, 30] });
-  }
 }
 
 function buildMap(el) {
@@ -81,10 +73,18 @@ function buildMap(el) {
     touchZoom: false,
     keyboard: false,
   }).setView(BERLIN_CENTER, BERLIN_ZOOM);
-  L.tileLayer("https://{s}.tile.openstreetmap.org/{z}/{x}/{y}.png", {
-    attribution: "© OpenStreetMap",
-    maxZoom: 18,
+  // CartoDB Voyager — clean, Google-Maps-ish base style.
+  L.tileLayer("https://{s}.basemaps.cartocdn.com/rastertiles/voyager/{z}/{x}/{y}{r}.png", {
+    attribution: "© OpenStreetMap · © CARTO",
+    subdomains: "abcd",
+    maxZoom: 19,
+  }).addTo(mapInstance);
+  // Transit overlay — OpenRailwayMap highlights S-/U-/Tram-Linien.
+  L.tileLayer("https://{s}.tiles.openrailwaymap.org/standard/{z}/{x}/{y}.png", {
+    attribution: "© OpenRailwayMap",
     subdomains: "abc",
+    maxZoom: 19,
+    opacity: 0.75,
   }).addTo(mapInstance);
   markerLayer = L.layerGroup().addTo(mapInstance);
 }