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/server.py

@@ -20,6 +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
 
 # In-memory CSRF state tokens for OAuth flows (token → True); cleared after use
 _oauth_states: set[str] = set()
@@ -427,6 +428,32 @@ async def save_activity(activity_id: str, payload: dict[str, Any]) -> JSONRespon
     return JSONResponse({"ok": True, "sidecar": str(sidecar_path)})
 
 
+@app.post("/api/activity/{activity_id}/recalculate-elevation")
+async def recalculate_elevation_endpoint(activity_id: str) -> JSONResponse:
+    """Replace GPS altitude with DEM terrain elevation and recompute gain/loss.
+
+    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).",
+        )
+    dd = _get_data_dir()
+    _check_id(activity_id)
+    try:
+        from bincio.extract.dem import recalculate_elevation
+        from bincio.render.merge import merge_one
+        result = recalculate_elevation(dd, activity_id, dem_url)
+        merge_one(dd, activity_id)
+        return JSONResponse(result)
+    except FileNotFoundError as e:
+        raise HTTPException(404, str(e))
+    except ValueError as e:
+        raise HTTPException(422, str(e))
+
+
 @app.post("/api/activity/{activity_id}/images")
 async def upload_image(activity_id: str, file: UploadFile = File(...)) -> JSONResponse:
     dd = _get_data_dir()