Phase 4: Web Push notifications (pywebpush + VAPID)

Backend: PushSubscription model/migration, VAPID config + `cli.py
generate-vapid-keys`, push_service.send_push_to_user (upsert-by-endpoint
subscribe/unsubscribe, auto-cleanup of expired 404/410 subscriptions),
/api/push/* router, and ConnectionManager now tracks connected user IDs
per room so chat.py can push only to offline members after broadcasting
to online ones.

Two test-infra bugs found and fixed along the way: send_push_to_user
takes the caller's AsyncSession and is awaited inline rather than fired
via asyncio.create_task with its own session (background tasks were
outliving the test event loop); and the ws_client fixture now uses
NullPool to eliminate a connection-pool checkout race that was failing
WS tests intermittently.

Frontend: service worker rebuilt with vite-plugin-pwa's injectManifest
strategy (custom src/sw.ts) so it can add push/notificationclick
handlers alongside the existing precaching and StaleWhileRevalidate
routes ported over from generateSW. New subscribe/unsubscribe flow
(lib/push.ts, api/push.ts) with a toggle in the account menu.

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

frontend/src/components/TopBar.css

@@ -94,3 +94,15 @@
 .top-bar-menu button[role='menuitem']:hover {
   background: var(--ds-surface-2);
 }
+
+.top-bar-menu button[role='menuitem']:disabled {
+  opacity: 0.5;
+  cursor: not-allowed;
+}
+
+.top-bar-menu-error {
+  color: var(--ds-danger);
+  font-size: 0.74rem;
+  padding: 2px 8px 6px;
+  max-width: 220px;
+}