feat(dashcam): async import with polling status

- Import now returns immediately with {status: 'started'}
- Backend runs copy in a daemon thread, tracks job state in _import_job
- New GET /api/dash/import/status endpoint for polling
- UI polls every 3s, shows progress bar until done
- Page reload mid-import resumes polling automatically
- Prevents double-import with 'busy' guard

Co-authored-by: Copilot <[email protected]>

app.py

@@ -97,21 +97,18 @@ def dash_devices():
 @app.route('/api/dash/import', methods=['POST'])
 def dash_import():
     body = request.get_json(silent=True)
-    if body is None:
-        relative_paths = None
-    elif isinstance(body, dict):
-        relative_paths = body.get("paths", [])
-    else:
-        relative_paths = []
-    summary = app_dash.import_dashcam(relative_paths=relative_paths)
-    status_code = (
-        200
-        if summary["status"] == "ok"
-        else 400
-        if summary["status"] == "invalid_selection"
-        else 409
-    )
-    return jsonify(summary), status_code
+    relative_paths = body.get("paths") if isinstance(body, dict) else None
+    started, err = app_dash.start_import_async(relative_paths=relative_paths)
+    if not started:
+        return jsonify({"status": "busy", "message": err}), 409
+    return jsonify({"status": "started"})
+
+@app.route('/api/dash/import/status')
+def dash_import_status():
+    job = app_dash.get_import_status()
+    if job is None:
+        return jsonify({"status": "none"})
+    return jsonify(job)
 
 @app.route('/api/dash/files')
 def dash_files():