Documentation

Migrate the nonce store (brig, DPoP replay protection) from Cassandra to PostgreSQL.

A nonce is issued on GET clientscid/nonce and consumed (lookup-and- delete) on the first access-token request. A replayed attempt must be rejected with 400 client-token-bad-nonce. This is verified in all three storage modes; the postgresql leg is the load-bearing proof that consume works with PostgreSQL as the sole source of truth.