gateway: converge, don't start/stop/reload

The gateway is a deployment (castle-gateway) — its lifecycle is the same
convergence as everything else, so the bespoke start/stop/reload verbs
(which still routed through the retired enable/disable path) are gone.

- CLI: `castle gateway` is now inspection only — bare or `status` shows
  the route table. Start/stop/reload it via `castle apply` (render routes
  + reload) or `castle restart castle-gateway`. Deletes the last users of
  `_service_enable`/`_service_disable`, so those are removed too.
- API: retire `POST /gateway/reload` (making routes live is `POST /apply`);
  `PUT /gateway/config` message + docstring now say apply, not deploy.
- UI: GatewayPanel's "Reload" button → "Apply" (useApply); drop the
  useGatewayReload hook; GatewaySettings points at `castle apply`.
- docs: `castle gateway reload|status` → `castle gateway` (status lens).

core 129 / cli 31 / api 59 pass; dashboard builds clean; live `castle
gateway` shows routes with no start/stop/reload; /gateway/reload removed
from the API.

docs/stacks/python-fastapi.md

@@ -454,4 +454,4 @@ castle program create my-service --stack python-fastapi --description "Does some
 ```
 
 See @docs/registry.md for manifest fields, castle.yaml structure,
-and the full service lifecycle (enable, logs, gateway reload).
+and the full service lifecycle (`castle apply`, logs).