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/edit/server.py

@@ -20,7 +20,7 @@ data_dir: Path | None = None
 site_url: str = "http://localhost:4321"
 strava_client_id: str = ""
 strava_client_secret: str = ""
-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
 
 # In-memory CSRF state tokens for OAuth flows (token → True); cleared after use
 _oauth_states: set[str] = set()
@@ -435,11 +435,7 @@ async def recalculate_elevation_endpoint(activity_id: str) -> JSONResponse:
     Requires --dem-url to be set when starting bincio edit.
     """
     if not dem_url:
-        raise HTTPException(
-            503,
-            "DEM URL not configured. "
-            "Pass --dem-url <api-url> to bincio edit (e.g. https://api.open-elevation.com).",
-        )
+        raise HTTPException(503, "DEM URL not configured.")
     dd = _get_data_dir()
     _check_id(activity_id)
     try: