* Add `nimble_totp`, `cloak` and `cloak_ecto` to project dependencies

* Setup Cloak-based secrets vault and create a dedicated Ecto type

* Add `totp_enabled|secret|last_used_at` fields to `User` schema

* Implement schema and stateless logic for TOTP recovery codes

* Implement core logic of TOTP auth

* Fix typos and improve style of doc comments

Co-authored-by: hq1 <hq@mtod.org>

* Fix moduledoc alignment

* Use more compact conditional expression

Co-authored-by: hq1 <hq@mtod.org>

* Disambiguate `I` as `7` when generating recovery codes (h/t @hq1)

* Fix a typo in runtime config error message

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Co-authored-by: hq1 <hq@mtod.org>