perf(gateway): scale request hot paths for 20k streams

Shard and singleflight hot-path caches, batch and prioritize candidate and usage lifecycle persistence, and extend database and pressure-test instrumentation for 20k concurrent streams.

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

@@ -1844,7 +1844,7 @@ fn usage_repositories_are_owned_by_contracts_and_driver_adapters() {
             "sql"
         )
         .len(),
-        26,
+        27,
         "all PostgreSQL usage SQL fragments should be owned by the adapter crate"
     );
 }