Implement SignalR infrastructure for real-time features (notifications, live updates) with Redis backplane for scalability.

Acceptance Criteria:

• Add Microsoft.AspNetCore.SignalR.StackExchangeRedis package
• Create NotificationHub with connection management
• Implement user-to-connection tracking via Redis
• Add JWT authentication for SignalR connections
• Configure Redis backplane for multi-instance scaling
• Create INotificationDispatcher interface and implementation
• Add connection status logging and monitoring
• Create frontend SignalR client wrapper with auto-reconnect
• Add useSignalR React hook for connection management

Technical Notes:

• Map hub at /hubs/notifications
• Use existing Redis instance from DISTRIBUTEDCACHEURL
• Support automatic transport negotiation (WebSocket → SSE → Long Polling)