Release a stale lock

Releases a lock whose owner ended completion_unknown, after review. A live lock is never released here; its owner releases it by finishing, and a request against a live lock answers 409 with LOCK_HELD. A repository lock requires permission to change system settings; a storage lock requires the same access a restore does, and the storage's volume need not exist any more. At startup, once no helper of the previous process remains, Sencho releases the repository lock of an interrupted provisioning, and, when restic's own locks are cleared, of interrupted captures, checks and restores itself; every storage lock waits for this call. Only a Full Admin API token may call it; any other scope is refused with 403 SCOPE_DENIED.