Fix Compression of DjangoConnector Backups

[Roukoswarf]

Description

The DjangoConnector returned a text-mode file, which made the compression code which expected binary as is returned from other connectors to crash with:

  File "/opt/infradesigner/.venv/lib/python3.14/site-packages/dbbackup/management/commands/dbbackup.py", line 143, in _save_new_backup
    compressed_file, filename = utils.compress_file(outputfile, filename)
                                ~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/infradesigner/.venv/lib/python3.14/site-packages/dbbackup/utils.py", line 271, in compress_file
    copyfileobj(inputfile, zipfile, settings.TMP_FILE_READ_SIZE)
    ~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.14/shutil.py", line 257, in copyfileobj
    fdst_write(buf)
    ~~~~~~~~~~^^^^^
  File "/usr/lib/python3.14/gzip.py", line 321, in write
    return self._buffer.write(data)
           ~~~~~~~~~~~~~~~~~~^^^^^^
TypeError: a bytes-like object is required, not 'str'

Tests don't seem to cover this combination, maybe I will have time to write tests for this, but for now I just needed this fixed for myself.

This also accidentally seems to fix an issue paramiko (SFTP Storage) was having where the os.stat(). Sizes were always larger on the target host vs the source, though the file seemed correct. Did not investigate much, but I suspect it also could be related to utf8 vs on-disk size.

Checklist

Please update this checklist as you complete each item:

• Tests have been developed for bug fixes or new functionality.
• The changelog has been updated, if necessary.
• Documentation has been updated, if necessary.
• GitHub Issues closed by this PR have been linked.

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