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

generate-vapid-keys.sh

@@ -0,0 +1,81 @@
+#!/bin/bash
+
+# VAPID Key Generation Script for Web Push Notifications
+# Generates VAPID (Voluntary Application Server Identification) keys required for push notifications
+
+echo "==========================================="
+echo "VAPID Key Generation for Inventarsystem"
+echo "==========================================="
+echo ""
+
+# Check if Python is available
+if ! command -v python3 &> /dev/null; then
+    echo "❌ Error: Python 3 is required but not installed."
+    exit 1
+fi
+
+# Check if pywebpush is installed
+echo "Checking for pywebpush..."
+python3 -c "import pywebpush" 2>/dev/null
+
+if [ $? -ne 0 ]; then
+    echo "Installing pywebpush..."
+    pip3 install pywebpush
+    if [ $? -ne 0 ]; then
+        echo "❌ Error: Failed to install pywebpush"
+        echo "Please install manually: pip3 install pywebpush"
+        exit 1
+    fi
+fi
+
+echo "✓ pywebpush is installed"
+echo ""
+
+# Generate VAPID keys
+echo "Generating VAPID keys..."
+python3 << 'EOF'
+from pywebpush import generate_keys
+
+try:
+    keys = generate_keys()
+    print("✓ VAPID Keys generated successfully!")
+    print("")
+    print("PUBLIC KEY (share with browsers):")
+    print(keys['public_key'])
+    print("")
+    print("PRIVATE KEY (keep secret!):")
+    print(keys['private_key'])
+    print("")
+    print("==========================================="
+    print("Add these to your environment variables:")
+    print("==========================================="
+    print("")
+    print("export VAPID_PUBLIC_KEY='" + keys['public_key'] + "'")
+    print("export VAPID_PRIVATE_KEY='" + keys['private_key'] + "'")
+    print("export VAPID_SUBJECT='mailto:admin@yourdomain.com'")
+    print("")
+    print("Or add to your .env file:")
+    print("")
+    print("VAPID_PUBLIC_KEY=" + keys['public_key'])
+    print("VAPID_PRIVATE_KEY=" + keys['private_key'])
+    print("VAPID_SUBJECT=mailto:admin@yourdomain.com")
+    print("")
+    print("⚠️  IMPORTANT: Keep the PRIVATE KEY secret!")
+    print("==========================================="
+    
+except Exception as e:
+    print(f"❌ Error generating VAPID keys: {e}")
+    exit(1)
+EOF
+
+if [ $? -eq 0 ]; then
+    echo ""
+    echo "✓ Next steps:"
+    echo "1. Copy the PUBLIC KEY to your browser-side code"
+    echo "2. Set the PRIVATE KEY in your server environment"
+    echo "3. Update config.json with your email address"
+    echo "4. Test with: curl http://localhost:5000/api/push/vapid-key"
+else
+    echo "❌ Error generating keys"
+    exit 1
+fi