feat(web): Orchestrator page + SSE hook + AUTOMATION nav group

New /orchestrator route. Paginated read-only event list with kind
filter (all|traffic|file), pause-stream toggle, in-window failure
badge ('X failures / 1h'), and an SSE-driven 'live' status pill.
Streamed rows prepend on top up to a 500-row in-memory cap.

Sidebar gains an AUTOMATION nav group; Orchestrator is the first
child. Future workers (mutator/prober activity) plug in as siblings.

decnet_web/src/routePrefetch.ts

@@ -22,6 +22,7 @@ const loaders: Record<string, Loader> = {
   '/config':         () => import('./components/Config'),
   '/swarm-updates':  () => import('./components/RemoteUpdates'),
   '/swarm/hosts':    () => import('./components/SwarmHosts'),
+  '/orchestrator':   () => import('./components/Orchestrator'),
 };
 
 const fired = new Set<string>();