0201 — Producers publish a routing_key attribute; groups opt in via binding rows; no binding means receive everything

Context. Routing was needed without producers addressing consumers directly, and without changing behavior for any consumer that existed before routing did.

Decision. message_log gains a routing_key TEXT column; Datastore.AppendMessage/WorkProducer.Produce take a routingKey string, with "" stored as SQL NULL rather than ''. A new binding(consumer_group, pattern, display) table (indexed on consumer_group) lets a group opt into only the events whose routing_key matches a pattern. A group with no binding rows receives everything. AppendMessage never reads binding — the producer writes one attribute and never learns a consumer exists.

Consequences. All pre-routing behavior is unchanged by default; filtering is strictly opt-in per group. The producer/consumer decoupling is total: the only coupling surface is the routing_key value and the binding rows that match it.