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>

index.html

@@ -7,6 +7,13 @@
     <meta name="description" content="HotelSync — современная PMS система для отелей" />
     <meta name="robots" content="noindex, nofollow" />
     <title>HotelSync</title>
+    <link rel="manifest" href="/manifest.json" />
+    <meta name="theme-color" content="#4F46E5" />
+    <meta name="mobile-web-app-capable" content="yes" />
+    <meta name="apple-mobile-web-app-capable" content="yes" />
+    <meta name="apple-mobile-web-app-status-bar-style" content="black-translucent" />
+    <meta name="apple-mobile-web-app-title" content="HotelSync" />
+    <link rel="apple-touch-icon" href="/icons/icon-180.png" />
     <link rel="preconnect" href="https://fonts.googleapis.com" />
     <link rel="preconnect" href="https://fonts.gstatic.com" crossorigin />
     <link href="https://fonts.googleapis.com/css2?family=Inter:wght@300;400;500;600;700&display=swap" rel="stylesheet" />