feat(notifications): wire publisher + async feature flag (GOO-173)

Phase 1 step 3 — make NotificationsPublisher available via DI and
introduce the NOTIFICATIONS_ASYNC_ENABLED flag that will gate the
listener cutover.

- NotificationsAsyncConfig: tiny injectable that reads
  NOTIFICATIONS_ASYNC_ENABLED (truthy: 1/true/yes/on, default disabled).
  Callers don't touch process.env directly; per-category rollout can be
  added later without churn in listeners.
- NotificationsModule providers/exports now include NotificationsPublisher
  and NotificationsAsyncConfig so listeners/handlers can inject them.
- Tests (14 specs on the flag + 4 on the publisher = 18 total green):
  * default disabled when unset
  * truthy parsing (1, true, TRUE, yes, on, padded)
  * falsy parsing (false, 0, no, off, empty, unknown)
  * describe() reports human-readable state

No call sites updated yet — next commit migrates the first listener
(PaymentCompletedListener pilot) onto the publisher with the flag check.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

apps/web/components/agents/agent-profile-client.tsx

@@ -267,7 +267,7 @@ export function AgentProfileClient({
   return (
     <div className="mx-auto max-w-7xl px-4 py-6 space-y-6">
       {/* ── Breadcrumb ── */}
-      <nav className="flex items-center gap-1.5 text-xs text-foreground-muted">
+      <nav aria-label="Breadcrumb" className="flex items-center gap-1.5 text-xs text-foreground-muted">
         <Link href="/" className="hover:text-foreground transition-colors">Trang chủ</Link>
         <span>/</span>
         <Link href="/agents" className="hover:text-foreground transition-colors">Môi giới</Link>