Scoping and concurrency

Conversation identity defaults to [sessionId, userId]. Add tenant metadata or provide a deterministic scope function when those values can collide.

Appends run in a transaction. With the default lock: 'advisory', the adapter acquires pg_advisory_xact_lock(hashtext(scopeKey)) before reading the current position and inserting messages. That serializes writers for the same scope while allowing unrelated conversations to proceed.

lock: 'none' removes that protection and can expose concurrent position conflicts. Choose it only when a queue, application mutex, or another database protocol guarantees one writer per conversation.

Scopes are not row-level security. Enforce tenant authorization independently, and consider Postgres RLS only with a deliberately configured connection/session strategy.