feat: PWA push notifications — service worker, VAPID, subscription management

- Add web-push VAPID backend with push_subscriptions table (migration 078)
- Backend push module with sendPushToUser/sendPushToRoom helpers
- Push routes: GET vapid-key, POST subscribe, DELETE unsubscribe
- Trigger push on new chat messages in direct/group rooms
- PWA manifest, service worker, and icons (72–512px)
- Frontend push lib: SW registration, subscribe/unsubscribe helpers
- Push API in api.ts, SW registered in main.tsx
- usePushSubscription hook for auto-subscribe on permission grant
- PushPermissionRow component in ChatWidget settings view

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/lib/api.ts

@@ -897,6 +897,15 @@ export const api = {
       req<{ ok: boolean }>('DELETE', `/api/hotels/${slug}/payment-gateways/${id}`),
   },
 
+  // ── Push notifications ────────────────────────────────────────────────────
+  push: {
+    getVapidKey: () => req<{ publicKey: string | null }>('GET', '/api/push/vapid-key'),
+    subscribe: (sub: { endpoint: string; p256dh: string; auth: string }) =>
+      req<{ ok: boolean }>('POST', '/api/push/subscribe', sub),
+    unsubscribe: (endpoint: string) =>
+      req<{ ok: boolean }>('DELETE', '/api/push/unsubscribe', { endpoint }),
+  },
+
   // Public widget API (no auth)
   widget: {
     getConfig: (slug: string) =>