Add SQLite backing store for payjoin sessions

Add SQLite backing store for payjoin sessions

Persist payjoin sender and receiver state in SQLite so interrupted
payjoin sessions can be resumed after the CLI exits. Add dedicated
tables for send and receive sessions, append-only event logs for
state replay, receiver pubkey lookup for sender sessions, and
seen-input tracking for replay protection.

This follows the intended async payjoin design by persisting session
state across interruptions. SQLite keeps the initial persistence
backend simple and builds on existing rusqlite support, at the cost
of a small payjoin-specific schema and serialization layer.