Implement Web Push Notifications system

- Add push_notifications.py module for managing subscriptions and sending notifications
- Create Flask API endpoints for subscription management (/api/push/*)
- Integrate push sending with existing notification system (_create_notification)
- Create frontend JavaScript library (push-notifications.js) for client-side subscription
- Update base.html template with manifest link and Service Worker registration
- Add manifest.json and service-worker.js for PWA support
- Add pywebpush dependency to requirements.txt
- Create generate-vapid-keys.sh script for VAPID key generation
- Add comprehensive documentation (WEB_PUSH_NOTIFICATIONS.md)

Features:
- Subscribe/unsubscribe users from push notifications
- Store subscriptions in MongoDB with automatic indexing
- Send push to individual users or all admins
- Notification deduplication and versioning
- Service Worker handles offline caching and push events
- Full PWA support for mobile app installation
- VAPID authentication for secure push communication

Web/templates/base.html

@@ -24,6 +24,7 @@
     <link rel="stylesheet" href="{{ url_for('static', filename='css/styles.css', v=ASSET_VERSION) }}">
     <link rel="stylesheet" href="{{ url_for('static', filename='css/planned_appointments.css', v=ASSET_VERSION) }}">
     <link rel="icon" href="{{ url_for('static', filename='favicon.ico') }}">
+    <link rel="manifest" href="{{ url_for('static', filename='manifest.json') }}">>
     <script src="https://cdn.jsdelivr.net/npm/bootstrap@5.3.0-alpha1/dist/js/bootstrap.bundle.min.js"></script>
     <script>
         (function () {
@@ -1752,5 +1753,28 @@
     <!-- Mobile compatibility scripts -->
     <script src="{{ url_for('static', filename='js/mobile_compatibility.js') }}"></script>
     <script src="{{ url_for('static', filename='js/ios_fixes.js') }}"></script>
+    
+    <!-- Service Worker Registration for Push Notifications -->
+    <script>
+        document.addEventListener('DOMContentLoaded', function() {
+            // Register service worker for push notifications and offline support
+            if ('serviceWorker' in navigator) {
+                navigator.serviceWorker.register('{{ url_for("static", filename="service-worker.js") }}')
+                    .then(function(registration) {
+                        console.log('Service Worker registered successfully:', registration);
+                        
+                        // Check if we have permission for push notifications
+                        if ('Notification' in window && Notification.permission === 'granted') {
+                            console.log('Push notifications already enabled');
+                        }
+                    })
+                    .catch(function(error) {
+                        console.warn('Service Worker registration failed:', error);
+                    });
+            } else {
+                console.log('Service Workers not supported in this browser');
+            }
+        });
+    </script>
 </body>
 </html>