fix(queue): let a shut-down queue be started again

`shutdown()` set `accepting = false` and `start()` refused for ever after, so
a queue was single-use. Any process that boots more than one app broke: a test
suite closing one harness and opening the next, a hot reload, a multi-tenant
host. The failure landed far from its cause -- the SECOND app to boot threw
WRN-QUEUE-CLOSED out of the dev server because an unrelated one had shut down
earlier in the same process. That is what turned six example-app security
tests red only when run alongside the rest of the suite.

Starting is an explicit intent to run, so it reopens the queue. `add()` keeps
its guard, so work offered to a queue that is shutting down is still refused.

Also migrates the example app's welcome-email queue to `defineQueue`, which
the new loader requires. It still used `defineJob`, so the loader refused it
and took the whole example app down -- 14 failures from one unmigrated file.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

examples/basic-app/app/queues/welcome-email.ts

@@ -1,13 +1,28 @@
-import { defineJob } from "@wrnexus/queue";
+import { defineQueue } from "@wrnexus/queue";
 
 export interface WelcomeEmailPayload {
   userId: number;
   email: string;
 }
 
-export default defineJob<WelcomeEmailPayload>({
+/*
+ * A file under app/queues must default-export defineQueue(...) -- the loader
+ * refuses anything else rather than registering a queue that would never run.
+ * This example used the older defineJob() shape and so failed to load, which
+ * took the whole example app down with it.
+ */
+export default defineQueue({
   name: "welcome-email",
-  async run(job) {
-    console.log(`Welcome email queued for ${job.data.email}`);
+  jobs: {
+    send: {
+      validate: (data: unknown): data is WelcomeEmailPayload =>
+        typeof data === "object" &&
+        data !== null &&
+        Number.isInteger((data as { userId?: unknown }).userId) &&
+        typeof (data as { email?: unknown }).email === "string",
+      run: async ({ email }: WelcomeEmailPayload) => {
+        console.log(`Welcome email queued for ${email}`);
+      },
+    },
   },
 });