feat: DEM-based elevation recalculation via edit drawer button

Adds a "Recalculate from terrain map (DEM)" button to the activity edit
drawer. On click it queries an Open-Elevation-compatible API to replace
GPS altitude with SRTM terrain data, applies 5m hysteresis, and updates
the activity's elevation stats and timeseries chart in place.

- bincio/extract/dem.py: lookup_elevations() (batched HTTP POST) +
  recalculate_elevation() (subsample → DEM → interpolate → hysteresis →
  patch activity JSON, timeseries JSON, index.json)
- POST /api/activity/{id}/recalculate-elevation on both serve and edit
  servers; serve endpoint is auth-gated and triggers merge + rebuild
- --dem-url flag (also DEM_URL env var) on bincio serve and bincio edit;
  logged at startup; missing URL returns a clear 503 with setup instructions
- /api/me response gains dem_configured bool
- EditDrawer: button with loading state, shows new ↑/↓ values on success

bincio/edit/cli.py

@@ -24,6 +24,8 @@ console = Console()
               help="Strava API client ID (enables Strava sync in the UI). Also reads STRAVA_CLIENT_ID env var.")
 @click.option("--strava-client-secret", default=None, envvar="STRAVA_CLIENT_SECRET",
               help="Strava API client secret. Also reads STRAVA_CLIENT_SECRET env var.")
+@click.option("--dem-url", default=None, envvar="DEM_URL",
+              help="Base URL of an Open-Elevation-compatible API (enables 'Recalculate elevation' button).")
 def edit(
     data_dir: Optional[str],
     port: int,
@@ -31,6 +33,7 @@ def edit(
     config_path: Optional[str],
     strava_client_id: Optional[str],
     strava_client_secret: Optional[str],
+    dem_url: Optional[str],
 ) -> None:
     """Start a local web UI for editing activity sidecar files.
 
@@ -69,11 +72,16 @@ def edit(
     srv.site_url = site_url
     srv.strava_client_id = strava_client_id or ""
     srv.strava_client_secret = strava_client_secret or ""
+    srv.dem_url = dem_url or ""
 
     if strava_client_id:
         console.print(f"Strava sync: [green]enabled[/green] (client {strava_client_id})")
     else:
         console.print("Strava sync: [yellow]disabled[/yellow] (pass --strava-client-id to enable)")
+    if dem_url:
+        console.print(f"DEM:         [green]enabled[/green] ({dem_url})")
+    else:
+        console.print("DEM:         [yellow]disabled[/yellow] (pass --dem-url to enable elevation recalculation)")
 
     uvicorn.run(srv.app, host="127.0.0.1", port=port, log_level="warning")