[MEDIA-012] Destination folder rules

[veryCrunchy]

Roadmap

• Task ID: MEDIA-012
• Priority: P1
• Imported status: todo

User problem

People who back up several Android media folders need control over the remote layout. Flattening every source into one destination loses useful album and application folder context, while forcing a hierarchy on everyone can make simple camera backup harder to browse.

Acceptance criteria

• Flat, year, year/month, and source-folder-preserving layouts have deterministic, previewable remote paths.
• The setup UI explains each layout and previews representative destination paths before enabling upload.
• Source-folder-preserving mode keeps each selected media folder under the configured Nextcloud destination without exposing local absolute paths or device-specific identifiers.
• Remote path segments are normalized safely and cannot escape the configured destination.
• Multiple source folders that normalize to the same remote path have an explicit, deterministic collision policy.
• Changing a layout affects newly queued uploads predictably and does not silently move or duplicate already uploaded originals.
• The selected rule survives restart, permission changes, and background scheduling.
• Deterministic tests cover nested folders, Unicode names, duplicate names, unsafe path segments, and layout changes.