feat(backend): Implement SignalR real-time communication infrastructure

Add complete SignalR infrastructure for real-time project collaboration and notifications with multi-tenant isolation and JWT authentication.

Changes:
- Created BaseHub with multi-tenant isolation and JWT authentication helpers
- Created ProjectHub for real-time project collaboration (join/leave, typing indicators)
- Created NotificationHub for user-level notifications
- Implemented IRealtimeNotificationService for application layer integration
- Configured SignalR in Program.cs with CORS and JWT query string support
- Added SignalRTestController for connection testing
- Documented hub endpoints, client events, and integration examples

Features:
- Multi-tenant isolation via automatic tenant group membership
- JWT authentication (Bearer header + query string for WebSocket)
- Hub endpoints: /hubs/project, /hubs/notification
- Project-level events: IssueCreated, IssueUpdated, IssueStatusChanged, etc.
- User-level notifications with tenant-wide broadcasting
- Test endpoints for validation

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

colaflow-api/src/ColaFlow.API/Services/IRealtimeNotificationService.cs

@@ -0,0 +1,15 @@
+namespace ColaFlow.API.Services;
+
+public interface IRealtimeNotificationService
+{
+    // Project-level notifications
+    Task NotifyProjectUpdate(Guid tenantId, Guid projectId, object data);
+    Task NotifyIssueCreated(Guid tenantId, Guid projectId, object issue);
+    Task NotifyIssueUpdated(Guid tenantId, Guid projectId, object issue);
+    Task NotifyIssueDeleted(Guid tenantId, Guid projectId, Guid issueId);
+    Task NotifyIssueStatusChanged(Guid tenantId, Guid projectId, Guid issueId, string oldStatus, string newStatus);
+
+    // User-level notifications
+    Task NotifyUser(Guid userId, string message, string type = "info");
+    Task NotifyUsersInTenant(Guid tenantId, string message, string type = "info");
+}