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]>

modules/app_dash.py

@@ -37,6 +37,10 @@ _local_video_state_lock = threading.Lock()
 _local_videos_in_use = {}
 _local_videos_deleting = set()
 
+# --- Async import job state ---
+_import_lock = threading.Lock()
+_import_job = None   # dict: {status, found, copied, skipped, errors, message} or None
+
 
 def begin_local_video_use(relative_path):
     with _local_video_state_lock:
@@ -569,5 +573,35 @@ def import_dashcam(
                 unmount_device(mount_point)
 
 
+def start_import_async(relative_paths=None):
+    """Start a background import thread.  Returns (True, None) if started,
+    or (False, message) if an import is already running."""
+    global _import_job
+    with _import_lock:
+        if _import_job and _import_job.get("status") == "running":
+            return False, "An import is already in progress."
+        _import_job = {
+            "status": "running",
+            "found": 0, "copied": 0, "skipped": 0,
+            "errors": [], "message": "",
+        }
+
+    def _run():
+        global _import_job
+        result = import_dashcam(relative_paths=relative_paths)
+        with _import_lock:
+            _import_job = result
+
+    t = threading.Thread(target=_run, daemon=True)
+    t.start()
+    return True, None
+
+
+def get_import_status():
+    """Return the current import job state dict, or None if no job has been run."""
+    with _import_lock:
+        return dict(_import_job) if _import_job else None
+
+
 if __name__ == "__main__":
     print(import_dashcam())