move clip.mp4 backend to clips folder (#11834)

* move clip.mp4 backend to clips folder

* improve caching

* fix check

frigate/video.py

@@ -300,7 +300,7 @@ class CameraWatchdog(threading.Thread):
                 for d in os.listdir(CACHE_DIR)
                 if os.path.isfile(os.path.join(CACHE_DIR, d))
                 and d.endswith(".mp4")
-                and not d.startswith("clip_")
+                and not d.startswith("preview_")
             ]
         )
         newest_segment_time = latest_segment