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

site/src/components/EditDrawer.svelte

@@ -24,6 +24,11 @@
   let confirmDelete = false;
   let deleting = false;
 
+  // Elevation recalculation from DEM
+  let recalculating = false;
+  let recalcStatus = '';
+  let recalcOk = false;
+
   // Form state
   let title = '';
   let sport: Sport = 'cycling';
@@ -119,6 +124,26 @@
       : [...hideStats, key];
   }
 
+  async function recalculateElevation() {
+    recalculating = true;
+    recalcStatus = '';
+    recalcOk = false;
+    try {
+      const res = await fetch(`${api}/recalculate-elevation`, { method: 'POST' });
+      const d = await res.json();
+      if (!res.ok) throw new Error(d.detail ?? await res.text());
+      recalcOk = true;
+      const gain = d.elevation_gain_m != null ? `↑ ${Math.round(d.elevation_gain_m)} m` : '';
+      const loss = d.elevation_loss_m != null ? `↓ ${Math.round(d.elevation_loss_m)} m` : '';
+      recalcStatus = [gain, loss].filter(Boolean).join('  ');
+    } catch (e: any) {
+      recalcStatus = e.message;
+      recalcOk = false;
+    } finally {
+      recalculating = false;
+    }
+  }
+
   async function deleteActivity() {
     if (!confirmDelete) { confirmDelete = true; return; }
     deleting = true;
@@ -264,6 +289,24 @@
         </div>
       </div>
 
+      <!-- Elevation recalculation -->
+      <div class="mb-4">
+        <p class="text-xs text-zinc-500 mb-2">Elevation</p>
+        <button
+          type="button"
+          class="w-full flex items-center justify-center gap-2 px-3 py-2 rounded-lg border border-zinc-700 text-xs text-zinc-400 hover:border-zinc-500 hover:text-zinc-200 transition-colors disabled:opacity-40"
+          disabled={recalculating}
+          on:click={recalculateElevation}
+        >
+          {recalculating ? 'Querying terrain data…' : '⛰ Recalculate from terrain map (DEM)'}
+        </button>
+        {#if recalcStatus}
+          <p class="text-xs mt-1.5 text-center" class:text-green-400={recalcOk} class:text-red-400={!recalcOk}>
+            {recalcStatus}
+          </p>
+        {/if}
+      </div>
+
       <!-- Flags -->
       <div class="flex gap-3 mb-2">
         <button