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.

docs/wiki/HTTP-API.md

@@ -173,6 +173,9 @@ curl -s "$API/api/subagents" | jq                  # background agents
 curl -s "$API/api/search?q=deploy" | jq            # cross-session search
 curl -s "$API/api/mcp-sync" | jq                    # preview MCP server sync (opt-in: 403 until mcpSyncEnabled is on)
 curl -s -X POST "$API/api/mcp-sync" | jq             # apply it: add missing servers to each CLI config, never edit/remove
+curl -s "$API/api/config-backups" | jq              # config snapshots: folder, retention, list (admin only in multi-user mode)
+curl -s -X POST "$API/api/config-backups" | jq      # back up the customized config files now
+curl -s -X POST "$API/api/config-backups/cfg-20261011-120000/restore" | jq   # put a snapshot back (after a pre-restore copy)
 
 # with ID set to a session id:
 curl -s "$API/api/sessions/$ID/last-response" | jq -r '.data.text'   # last answer, from the transcript (claude, codex, deepseek)