fix(backups): honor user-defined S3 prefix

[imajus]

Summary

Restores the user-defined backup prefix as authoritative while preserving the retention-cleanup fix from #3919.

PR #3919 fixed a real bug in keepLatestNBackups (issue #3855) where path.join() mangled rclone's :s3:bucket remote syntax, causing every scheduled run to delete all backups. Alongside that fix it bundled an unrelated structural change: prepending the auto-generated appName to every backup path (bucket/{appName}/{prefix}/...).

That second change is what issue #4051 reports:

• The user-defined prefix is no longer authoritative — appName always sits in front of it.
• Pre-v0.28.5 backups stored at bucket/{prefix}/... are orphaned by the new retention logic.
• Bucket-layout assumptions break silently (e.g. a previously-private path can now land alongside a public one with the same name).
• Shipped without a changelog entry or migration path.

This PR keeps the actual retention fix from #3919 and removes the appName insertion across all backup writers and the cleanup paths.

After this change, paths are again:

bucket/{userPrefix}/{filename}                      # all DB / web-server backups
bucket/{userPrefix}/{filename}                      # volume backups, prefix set
bucket/{volume-app-name}/{filename}                 # volume backups, prefix empty (fallback unchanged)