Implement client cost attribution

README.md

@@ -32,8 +32,22 @@ uv run finhub ops-costs tests/fixtures/hosteurope.csv
 Cross-hub coupling (`--emit`) posts non-secret progress events to dev-hub when
 `STATE_HUB_API` is reachable.
 
+## Client cost attribution
+
+HostEurope CSV rows may include `client_id`, `application_id`, and
+`app_instance_id`. All three must be present together. Fin-hub validates them
+as external identifiers and derives the stable key
+`client:<client_id>|app:<application_id>|instance:<app_instance_id>`; callers
+must not invent a second key format. Rows without all three columns remain
+explicitly unattributed for backward compatibility.
+
+The SQLite ledger adds the attribution columns automatically when an existing
+ledger is opened. The `ops-costs` report retains its per-service view and adds
+an `attributions` view separated by currency. Client and application identity
+remain authoritative outside fin-hub.
+
 ## Related Workplans
 
 - `the-custodian/workplans/CUST-WP-0025-fos-hub-bootstrap.md` — umbrella
 - `canon/constitution/bootstrap-protocol_v0.1.md` — funding and roles
-- `canon/projects/railiance/business-model-canvas_v0.1.md` — monetization path
+- `canon/projects/railiance/business-model-canvas_v0.1.md` — monetization path