Add Web Push notifications for backgrounded/frozen browser tabs

The SSE + in-page toast/sound system only works while the tab's JS is
actually running — Chrome (and other browsers) freeze a backgrounded
tab's JS after a stretch of inactivity to save power, so notifications
silently stop regardless of how correct the SSE/toast code is. A Service
Worker is the only mechanism that keeps receiving events independent of
the tab's own lifecycle, so this adds a real Web Push pipeline:

- New `push_subscriptions` table (one row per browser/device a user has
  subscribed from; endpoint is unique so re-subscribing overwrites
  rather than accumulating stale rows).
- VAPID keypair config (lib/push/vapid.ts) — reads
  VAPID_PUBLIC_KEY/VAPID_PRIVATE_KEY/VAPID_SUBJECT from the environment;
  push is silently disabled (never throws) if they're unset, matching
  this codebase's existing "an optional integration outage must never
  break the core flow" pattern (see the LDAP auth comment).
- public/sw.js: a minimal service worker — `push` -> showNotification,
  `notificationclick` -> focus or open the ticket.
- API routes: GET /api/push/vapid-public-key (client needs it to call
  pushManager.subscribe), POST/DELETE /api/push/subscribe.
- lib/push/client.ts: registers the service worker and subscribes,
  called from the notification bell after granting permission and again
  on mount for returning users who already granted it.
- lib/tickets/service.ts: appendMessage() now also fires a push (fire-
  and-forget, never awaited by the caller) to every admin plus the
  ticket's assignee whenever a *customer* message arrives — same "who
  should know" rule as the in-page toast (lib/tickets/visibility.ts).
  Failed sends are inspected: a 404/410 (push service no longer
  recognizes the subscription) prunes the row; anything else is just
  logged, since it might be transient.

Verified server-side end-to-end on this VM: saved a subscription via the
API, created a customer ticket, and confirmed the push attempt actually
fires (web-push validated and rejected a deliberately-malformed test
key, proving the send path is wired correctly) without blocking or
crashing ticket creation. Couldn't verify the full real-browser
subscribe-and-receive path or an actual OS popup from here — this VM has
no desktop/notification service, and headless Chromium's Notification
permission can't be reliably granted in this sandbox (unrelated to the
app code); that last mile needs verifying on a real machine.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012o9j9RezxbZVKQMrB7oRLY

public/sw.js

@@ -0,0 +1,39 @@
+// Web Push service worker. Deliberately minimal — no offline caching, no
+// asset interception. Its only job is to keep receiving push events even
+// when the page's own tab is frozen or closed, and turn them into an OS
+// notification.
+
+self.addEventListener("push", (event) => {
+  let payload = { title: "top-tickets", body: "", url: "/dashboard" };
+  try {
+    if (event.data) payload = { ...payload, ...event.data.json() };
+  } catch {
+    // malformed payload — fall back to the generic notification above
+  }
+
+  event.waitUntil(
+    self.registration.showNotification(payload.title, {
+      body: payload.body,
+      icon: "/icon.svg",
+      data: { url: payload.url },
+    }),
+  );
+});
+
+self.addEventListener("notificationclick", (event) => {
+  event.notification.close();
+  const url = event.notification.data && event.notification.data.url ? event.notification.data.url : "/dashboard";
+
+  event.waitUntil(
+    (async () => {
+      const clientsList = await self.clients.matchAll({ type: "window", includeUncontrolled: true });
+      const existing = clientsList.find((c) => "focus" in c);
+      if (existing) {
+        await existing.navigate(url);
+        await existing.focus();
+        return;
+      }
+      await self.clients.openWindow(url);
+    })(),
+  );
+});