0305 — One queue-state query is the only derivation of the DB-truth health numbers

Context. Consumer health was previously visible only by querying Postgres directly (the ad hoc just lag recipe), and several new surfaces — snapshot, debug readout, OTel gauges — were about to need the same numbers at once.

Decision. One datastore method computes, live, per (group, topic): backlog (head - committed), the claimed - committed inflight gap, ready/inflight/dead exception counts, oldest-unacked age, and open-lease count. Everything else reads through this one query; no other code re-derives any of these numbers.

Consequences. One mechanism per fact: the query is DB-truth, and every consumer of it (snapshot, readout, gauges) agrees by construction. Each number is an early warning for a distinct failure mode — backlog for lag, dead exceptions for true failures, oldest-unacked age for a single stuck message, open leases for a crashed consumer that never committed — so the query is the operational surface, not an internal detail.