feat(config): timestamped config backups with a configurable folder and retention

The files that hold a user's customization (settings.json, clis.json, custom-model-hosts.json,
webhook.json, intents.json, linked-cases.json, push-keys.json, users.json, .env) are plain JSON
in the data dir, rewritten by the server, the Settings UI and the CLI, with no copy to go back to.

A snapshot is taken at startup and whenever the tracked files' content changes (checked every
five minutes, and only written when a digest of the file set differs from the latest snapshot's),
into `<data dir>/backups/config` by default or any absolute folder. Retention keeps the newest
configBackupKeepCount (1-500) and, when configBackupKeepDays > 0, drops older ones; it only ever
deletes folders it made (name pattern and a parseable manifest) and never the newest.

GET/POST /api/config-backups list and back up now; POST .../:id/restore verifies every file against
the manifest's SHA-256, takes a pre-restore snapshot, then replaces files atomically with mode
0600, writing only allowlisted names. Snapshots are 0700 folders of 0600 files and all routes are
admin only in multi-user mode. Settings -> System gets the controls and a Restore list.

.changeset/18dce807.md

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+---
+"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.