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/src/control/tests/public_support.rs

@@ -461,6 +461,11 @@ fn classifies_users_me_routes_as_public_support_route() {
             "/api/users/me/available-models",
             "available_models",
         ),
+        (
+            http::Method::GET,
+            "/api/users/me/routing-groups",
+            "routing_groups",
+        ),
         (
             http::Method::GET,
             "/api/users/me/vscodex/devices",