fix: default DEM URL to api.open-elevation.com

No configuration needed out of the box; --dem-url only required to
override the default with a self-hosted or alternative endpoint.

bincio/serve/server.py

@@ -158,7 +158,7 @@ webroot: Path | None = None    # nginx webroot — when set, trigger full rebuil
 strava_client_id: str = ""
 strava_client_secret: str = ""
 public_url: str = ""   # e.g. "https://yourdomain.com" — used for OAuth redirect URIs
-dem_url: str = ""      # Open-Elevation-compatible API base URL; empty = feature disabled
+dem_url: str = "https://api.open-elevation.com"  # Open-Elevation-compatible API base URL
 _db = None  # sqlite3.Connection, opened lazily
 
 
@@ -1279,11 +1279,7 @@ async def recalculate_elevation_endpoint(
     user = _require_user(bincio_session)
     _check_id(activity_id)
     if not dem_url:
-        raise HTTPException(
-            503,
-            "DEM URL not configured. "
-            "Pass --dem-url <api-url> to bincio serve (e.g. https://api.open-elevation.com).",
-        )
+        raise HTTPException(503, "DEM URL not configured.")
     dd = _get_data_dir() / user.handle
     if not (dd / "activities" / f"{activity_id}.json").exists():
         raise HTTPException(404, "Activity not found")