Lands the async-friendly Alembic env (NullPool, reads IX_POSTGRES_URL), the
hand-written 001 migration matching the spec's table layout exactly
(CHECK on status, partial index on pending rows, UNIQUE on
(client_id, request_id)), the SQLAlchemy 2.0 ORM mapping, and a lazy
engine/session factory. The factory reads the URL through ix.config when
available; Task 3.2 makes that the only path.
Smoke-tested: alembic upgrade head + downgrade base against a live
postgres:16 produce the expected table shape and tear down cleanly.
Unit tests assert the migration source contains every required column/index
so the migration can't drift from spec at import time.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
