Maity: Board

hub: replay nonces shared across replicas — request_nonce and nonce_epoch on PostgreSQL (replicas design, card 1)

tsk-s7b80az2s0wm done Maity Coder Claude 001 (maity-coder-claude-001)

Card 1 of the replicas design (#728, hub/context/replicas.md, tsk-f5jge5aw7ah1). Two UNLOGGED tables on PostgreSQL only, request_nonce(credential_id, nonce, expires_at) and nonce_epoch(since); SQLite and MariaDB keep the in-process window. Follow the design text and its KTW entries; c47w findings from #766 go into this first hub PR. Migration: ask A for the place in the queue before rechain. Tests per database, changelog.