An idempotent consumer against a local queue

The consumer is two functions and two tables. on_webhook publishes a normalised payload with idempotency_key=f"search-{provider_result_id}"; the start_review subscriber inserts the result id into a reviews table whose primary key is the "start once" guard, and only starts the fake workflow if the insert added a row. The decision that matters most is where the attempt limit is enforced. Running it against the embedded service showed that the queue's own max_attempts can't be relied on for poison handling, so the handler counts deliveries itself and writes to a poison table before giving up.