fix(dashcam): prevent site freeze during import/convert

- _convert_bg now runs in real threading.Thread (not gevent greenlet)
  so ffmpeg proc.wait() doesn't block gevent event loop
- _device_lock released before copy loop; only held for mount/unmount
  so /api/dash/files and other routes don't block during long copies
- Date now shows time (Aug 9, 10:30 PM) on mobile video rows
- Spacing added above converting spinner badge

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

modules/app_video.py

@@ -2,6 +2,7 @@
 import os
 import subprocess
 import time
+import threading
 
 import gevent
 from gevent import sleep
@@ -35,7 +36,9 @@ def queue_convert(relative_path):
         if relative_path in _converting:
             return
         _converting.add(relative_path)
-    gevent.spawn(_convert_bg, relative_path)
+    gevent.sleep(0)  # yield before spawning thread
+    t = threading.Thread(target=_convert_bg, args=(relative_path,), daemon=True)
+    t.start()
 
 
 def _convert_bg(relative_path):
@@ -59,8 +62,7 @@ def _convert_bg(relative_path):
                 tmp_path,
             ]
             proc = subprocess.Popen(cmd, stdout=subprocess.DEVNULL, stderr=subprocess.DEVNULL)
-            while proc.poll() is None:
-                sleep(1)
+            proc.wait()  # real thread — blocking wait is fine
             if proc.returncode == 0 and os.path.isfile(tmp_path) and os.path.getsize(tmp_path) > 0:
                 os.replace(tmp_path, mp4_path)
                 # Delete original now that MP4 is ready