Three community-requested features:
1. CLI password reset: `backupx reset-password --username admin --password xxx`
Docker users can run via `docker exec`. No full app init needed.
2. Audit logging: async fire-and-forget audit trail for all key operations
(login, CRUD on tasks/targets/records, settings changes).
New UI page at /audit with category filter and pagination.
3. Multi-source path backup: file backup tasks now support multiple source
directories packed into a single tar archive. Backward compatible with
existing single sourcePath field.
