Fix Web Mercator distance distortion in stop clustering

Clustering was computing distances in EPSG:3857 (Web Mercator), which
inflates distances by ~78% at Copenhagen's latitude (55.7°N). This made
the 100m threshold actually merge stops within only 56m, and all reported
distances were inflated.

Now reprojects to EPSG:25832 (UTM 32N) for accurate meter distances,
clusters there, converts centroids back to EPSG:3857 for plotting.

copenhagen/scripts/render.py

@@ -169,8 +169,16 @@ def classify_station(r):
     return None
 
 
+DISTANCE_CRS = "EPSG:25832"  # UTM 32N — accurate meters for Copenhagen
+
+
 def cluster_stops(coords, threshold_m):
-    """Cluster (x, y) coordinates within threshold_m; return list of centroids."""
+    """Cluster (x, y) coordinates in EPSG:3857 within threshold_m (real meters).
+
+    Reprojects to UTM 32N for accurate distance computation, clusters, then
+    returns centroids in EPSG:3857 for plotting.
+    """
+    from pyproj import Transformer
     from scipy.cluster.hierarchy import fcluster, linkage
     from scipy.spatial.distance import pdist
 
@@ -178,14 +186,21 @@ def cluster_stops(coords, threshold_m):
     if len(coords) <= 1:
         return coords.tolist() if len(coords) else []
 
-    dists = pdist(coords)
+    # reproject to UTM for accurate distances
+    to_utm = Transformer.from_crs(TARGET_CRS, DISTANCE_CRS, always_xy=True)
+    back = Transformer.from_crs(DISTANCE_CRS, TARGET_CRS, always_xy=True)
+    utm = np.array([to_utm.transform(x, y) for x, y in coords])
+
+    dists = pdist(utm)
     links = linkage(dists, method="single")
     labels = fcluster(links, t=threshold_m, criterion="distance")
 
     centroids = []
     for label in set(labels):
-        members = coords[labels == label]
+        members = utm[labels == label]
-        centroids.append(members.mean(axis=0).tolist())
+        ux, uy = members.mean(axis=0)
+        cx, cy = back.transform(ux, uy)
+        centroids.append((cx, cy))
     return centroids