Settings -> System -> Config backups gains a Compare view: a backup against the
running config, a backup against the one before it, or any two backups. Each
differing file lists its changed keys as before/after, plus files that appeared
or vanished. GET /api/config-backups/diff?from=&to= serves it (`to` defaults to
`current`; admin only in multi-user mode).
Credential values are never returned: .env, users.json and push-keys.json report
names only, and any other key that looks like a token, secret, password,
credential or key is reported as changed without its value. A non-JSON file
reports only that it differs, a file is cut at 200 changes, and a self-refreshed
discovery timestamp does not count as a difference.
Devvyn
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