chore(docker): persist converted videos via a named volume

ConvertedPath defaults to /app/converted on the container's ephemeral writable layer,
so mp4s were lost when the container was recreated while the DB still referenced them.
Mirror the existing thumbnails/previews mounts with a converted volume.

docker-compose.yml

@@ -26,6 +26,7 @@
      - "./storageImages:/diary:ro"
      - "thumbs:/app/thumbnails"
      - "previews:/app/previews"
+     - "converted:/app/converted"
   
   database:
     #image: tensorchord/pgvecto-rs:pg17-v0.4.0
@@ -69,3 +70,4 @@ volumes:
   pg_dbdata:
   thumbs:
   previews:
+  converted: