chore: support S3 storage when creating snapshots (beheer#22675)

Changelog

• Add setting S3_MEDIA_ROOT controlled by a same-named environment variable.
• Use S3_MEDIA_ROOT in snapshots for tenants that use object storage.

Instructions

1. Run management command create_snapshot with the schema name of a tenant where custom_file_storage="object_storage".

Related MRs

• !118 (merged).

Notes to reviewers

I left questions for you in the thread; could you check these?