feat: add selectable routing groups and composite billing

Support per-model provider enablement and compact model editing. Capture request-time billing factors, charge customer costs separately, and preserve historical statistics without backfills.

apps/aether-gateway/tests/architecture/sql_and_data.rs

@@ -1449,7 +1449,7 @@ fn usage_repositories_are_owned_by_contracts_and_driver_adapters() {
             "sql"
         )
         .len(),
-        27,
+        29,
         "all PostgreSQL usage SQL fragments should be owned by the adapter crate"
     );
 }