`recovery().enable()` refuses only when a key backup exists. It then makes
a new secret-storage key and sets it as the account's default regardless
of the default already there, so an account with secret storage but no
backup (set up in another client, backup since deleted) would have its
default key replaced and its other clients' recovery key locked out.
`recover_or_enable` now asks `secret_storage().is_enabled()` before
`enable` and returns an error instead, which `ensure_recovery` logs at
WARN with the account and the passphrase's store path. The decision is
behind a small trait over the matrix-sdk calls so a fake can pin it: with
no backup and secret storage present, neither `enable` nor `recover` runs.
Review: argus on #5188.
aece1c26d0