WEBUSAGE-2c per-app consumer hooks: how an app reads its own usage back

Per-app consumer hooks: the path by which an app READS its own usage data back, as opposed to a human opening a dashboard. Motivating case is the one already on record - coder-pluto-cc needed a route ranking for pluto and got a hand-run query from me (PLUTO-40, 2026-07-17). Scope: decide the shape (a queryable HTTP read endpoint on the ingest service vs a scheduled push vs direct DB role per app), and note that the ingest currently exposes /v1/usage write-only plus /healthz - adding a read route to it puts a cross-tenant read on a service whose only gate today is a per-app WRITE bearer, which is why this waits on WEBUSAGE-6. Explicitly NOT the dashboard (WEBUSAGE-7): that is for people, this is for code.