feat: checkbox batch trip delete on the Pi (local-only)

- Pi history page gets a select checkbox column + 'Delete selected' button
  (mirrors the server UI)
- POST /api/trips/delete batch endpoint
- app_sql.delete_trips_batch() + deleted_trip_markers table so a later trip
  rebuild can't resurrect locally-deleted trips
- Deletion is local-only: raw tracking rows are kept and the remote server
  is never notified (it keeps its archival copy recomputed from uploaded rows)

templates/history.html

@@ -15,8 +15,11 @@
     {# ── Trip list ──────────────────────────────────────────────────── #}
     <div class="col-12 col-xl-5">
         <div class="card shadow-sm">
-            <div class="card-header py-2">
+            <div class="card-header py-2 d-flex justify-content-between align-items-center">
                 <h6 class="text-primary fw-bold m-0">Recorded trips</h6>
+                <button id="delete-selected-btn" class="btn btn-sm btn-outline-danger" type="button" hidden title="Delete selected trips">
+                    <i class="fas fa-trash fa-fw"></i> Delete selected
+                </button>
             </div>
             <div class="card-body p-0">
                 <div id="trip-loading" class="text-center text-muted py-4">
@@ -27,6 +30,7 @@
                 {{ data_table(
                     "trip-list",
                     [
+                        {"key": "", "label": "", "sortable": false, "class": "text-center"},
                         {"key": "started", "label": "Started", "sortable": true},
                         {"key": "distance", "label": "km", "sortable": true},
                         {"key": "duration", "label": "Time", "sortable": true},
@@ -39,6 +43,7 @@
                     deletable=true
                 ) }}
                 <script>document.getElementById('trip-list').dataset.deleteUrl = '/api/trip/';</script>
+                <script>document.getElementById('trip-list').dataset.batchDeleteUrl = '/api/trips/delete';</script>
             </div>
         </div>
     </div>