feat: add postgres user engine store

docs/postgres-durable-store-consumer-requirements.md

@@ -301,9 +301,15 @@ values, profile values, prepared-account matches, and access-profile defaults.
 Adapters must avoid logging payloads and should use `record_counts` or other
 redacted diagnostics for observability.
 
+USER-WP-0018 adds `PostgresUserEngineStore`, a dependency-free adapter that
+accepts a provider-supplied DB-API or psycopg-like connection. It writes generic
+records through `StoreRecord`, keeps audit and outbox payloads in their
+dedicated bootstrap tables, applies the bootstrap SQL through `migrate`, and
+uses the shared conformance harness with a fake Postgres connection for local
+unit coverage.
+
 Likely future follow-up work should be:
 
-- Add a Postgres adapter behind the existing store boundary.
 - Add provider-backed conformance tests for locking, uniqueness races,
   migration readiness, outbox claiming, redacted diagnostics, and restore
   validation.