fix(backend): Move McpNotificationHub to Infrastructure layer to fix dependency inversion violation

Fixed compilation error where Infrastructure layer was referencing API layer (ColaFlow.API.Hubs).
This violated the dependency inversion principle and Clean Architecture layering rules.

Changes:
- Moved McpNotificationHub from ColaFlow.API/Hubs to ColaFlow.Modules.Mcp.Infrastructure/Hubs
- Updated McpNotificationHub to inherit directly from Hub instead of BaseHub
- Copied necessary helper methods (GetCurrentUserId, GetCurrentTenantId, GetTenantGroupName) to avoid cross-layer dependency
- Updated McpNotificationService to use new namespace (ColaFlow.Modules.Mcp.Infrastructure.Hubs)
- Updated Program.cs to import new Hub namespace
- Updated McpNotificationServiceTests to use new namespace
- Kept BaseHub in API layer for ProjectHub and NotificationHub

Architecture Impact:
- Infrastructure layer no longer depends on API layer
- Proper dependency flow: API -> Infrastructure -> Application -> Domain
- McpNotificationHub is now properly encapsulated within the MCP module

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

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

colaflow-api/tests/Modules/Mcp/ColaFlow.Modules.Mcp.Tests/Services/McpNotificationServiceTests.cs

@@ -1,5 +1,5 @@
-using ColaFlow.API.Hubs;
 using ColaFlow.Modules.Mcp.Application.DTOs.Notifications;
+using ColaFlow.Modules.Mcp.Infrastructure.Hubs;
 using ColaFlow.Modules.Mcp.Infrastructure.Services;
 using Microsoft.AspNetCore.SignalR;
 using Microsoft.Extensions.Logging;