Handle unreadable archives and adjust cron job configuration

- Skip page count assignment for corrupt or incompatible archives in comic processing.
- Update `Dockerfile` to append crontab entries to `/etc/crontab` instead of `/etc/cron.daily/cbreader`.
- Modify crontab entry to use the virtual environment's Python binary and include the `root` user for execution.

cbreader/crontab

@@ -1 +1 @@
-*/15    *       *       *       *       python /src/manage.py scan_comics --settings=cbreader.settings.base
+*/15 * * * * root /src/.venv/bin/python /src/manage.py scan_comics --settings=cbreader.settings.base