How to sync KeePassium via FTP/SFTP

FTP/SFTP servers are not natively supported by the iOS, but can be integrated with the Files app using third-party apps.

Secure ShellFish by Anders Borum (free)
FE File Explorer Pro (paid app)
FileBrowser by Stratospherix (paid app) — works well online and supports offline access, too. Once you are back online, open FileBrowser to upload local changes. Conflicting files are stored as numbered copies, so you can manually merge them in KeePass.
SFTP File Provider (paid app) — rather slow and reportedly does not work offline.

If you find something that works (or does not work) for you, please let us know.