feat(config-backup): compare backups with each other and with the running config

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.

.changeset/18dce807.md

@@ -2,4 +2,4 @@
 "aicodeman": minor
 ---
 
-Config backups: timestamped snapshots of the files that hold your customization (settings, CLI overrides, custom endpoints, webhook, linked cases and, in multi-user mode, accounts), taken at startup and whenever one changes. On by default into `~/.codeman/backups/config`; the folder and the retention (keep N backups, drop older than N days) are configurable in Settings → System, with Back up now and Restore. Retention only deletes folders Codeman made, never the newest backup.
+Config backups: timestamped snapshots of the files that hold your customization (settings, CLI overrides, custom endpoints, webhook, linked cases and, in multi-user mode, accounts), taken at startup and whenever one changes. On by default into `~/.codeman/backups/config`; the folder and the retention (keep N backups, drop older than N days) are configurable in Settings → System, with Back up now and Restore. Retention only deletes folders Codeman made, never the newest backup. A comparison view shows what changed between two backups, or between a backup and the running config, without ever showing a credential's value.