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/serve/cli.py

@@ -21,10 +21,11 @@ console = Console()
 @click.option("--max-users", default=None, type=int, help="Override max users for this instance (0 = unlimited; updates the DB setting)")
 @click.option("--public-url", default=None, envvar="PUBLIC_URL", help="Public base URL (e.g. https://yourdomain.com). Required for Strava OAuth to work behind a reverse proxy.")
 @click.option("--webroot", default=None, type=click.Path(), help="Nginx webroot (e.g. /var/www/bincio). When set, uploads trigger a full Astro build + rsync so new activity pages are immediately accessible without a git push.")
+@click.option("--dem-url", default=None, envvar="DEM_URL", help="Base URL of an Open-Elevation-compatible API (enables 'Recalculate elevation' button in the edit drawer).")
 def serve(data_dir: str, site_dir: Optional[str], host: str, port: int,
           strava_client_id: Optional[str], strava_client_secret: Optional[str],
           max_users: Optional[int], public_url: Optional[str],
-          webroot: Optional[str]) -> None:
+          webroot: Optional[str], dem_url: Optional[str]) -> None:
     """Start the bincio multi-user application server.
 
     Handles auth, user management, and write operations.
@@ -58,6 +59,8 @@ def serve(data_dir: str, site_dir: Optional[str], host: str, port: int,
         srv.public_url = public_url
     if webroot and site_dir:
         srv.webroot = Path(webroot).expanduser().resolve()
+    if dem_url:
+        srv.dem_url = dem_url
 
     db = open_db(dd)
     current_limit = get_setting(db, "max_users")
@@ -74,6 +77,8 @@ def serve(data_dir: str, site_dir: Optional[str], host: str, port: int,
         console.print(f"  Users: [yellow]max {current_limit}[/yellow]")
     else:
         console.print(f"  Users: [dim]unlimited[/dim]")
+    if dem_url:
+        console.print(f"  DEM:   [cyan]{dem_url}[/cyan]")
     console.print()
 
     log_config = uvicorn.config.LOGGING_CONFIG.copy()