feat(backend): Implement SignalR Real-Time Notifications for MCP - Story 5.12

Implemented comprehensive real-time notification system using SignalR to notify
AI agents and users about PendingChange status updates.

Key Features Implemented:
- McpNotificationHub with Subscribe/Unsubscribe methods
- Real-time notifications for all PendingChange lifecycle events
- Tenant-based isolation for multi-tenancy security
- Notification DTOs for structured message formats
- Domain event handlers for automatic notification sending
- Comprehensive unit tests for notification service and handlers
- Client integration guide with examples for TypeScript, React, and Python

Components Created:
1. SignalR Hub:
   - McpNotificationHub.cs - Central hub for MCP notifications

2. Notification DTOs:
   - PendingChangeNotification.cs (base class)
   - PendingChangeCreatedNotification.cs
   - PendingChangeApprovedNotification.cs
   - PendingChangeRejectedNotification.cs
   - PendingChangeAppliedNotification.cs
   - PendingChangeExpiredNotification.cs

3. Notification Service:
   - IMcpNotificationService.cs (interface)
   - McpNotificationService.cs (implementation using SignalR)

4. Event Handlers (send notifications):
   - PendingChangeCreatedNotificationHandler.cs
   - PendingChangeApprovedNotificationHandler.cs
   - PendingChangeRejectedNotificationHandler.cs
   - PendingChangeAppliedNotificationHandler.cs
   - PendingChangeExpiredNotificationHandler.cs

5. Tests:
   - McpNotificationServiceTests.cs - Unit tests for notification service
   - PendingChangeCreatedNotificationHandlerTests.cs
   - PendingChangeApprovedNotificationHandlerTests.cs

6. Documentation:
   - signalr-mcp-client-guide.md - Comprehensive client integration guide

Technical Details:
- Hub endpoint: /hubs/mcp-notifications
- Authentication: JWT token via query string (?access_token=xxx)
- Tenant isolation: Automatic group joining based on tenant ID
- Group subscriptions: Per-pending-change and per-tenant groups
- Notification delivery: < 1 second (real-time)
- Fallback strategy: Polling if WebSocket unavailable

Architecture Benefits:
- Decoupled design using domain events
- Notification failures don't break main flow
- Scalable (supports Redis backplane for multi-instance)
- Type-safe notification payloads
- Tenant isolation built-in

Story: Phase 3 - Tools & Diff Preview
Priority: P0 CRITICAL
Story Points: 3
Completion: 100%

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

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

colaflow-api/src/Modules/Mcp/ColaFlow.Modules.Mcp.Infrastructure/Extensions/McpServiceExtensions.cs

@@ -7,6 +7,7 @@ using ColaFlow.Modules.Mcp.Infrastructure.BackgroundServices;
 using ColaFlow.Modules.Mcp.Infrastructure.Middleware;
 using ColaFlow.Modules.Mcp.Infrastructure.Persistence;
 using ColaFlow.Modules.Mcp.Infrastructure.Persistence.Repositories;
+using ColaFlow.Modules.Mcp.Infrastructure.Services;
 using Microsoft.AspNetCore.Builder;
 using Microsoft.EntityFrameworkCore;
 using Microsoft.Extensions.Configuration;
@@ -44,6 +45,9 @@ public static class McpServiceExtensions
         services.AddScoped<IMcpApiKeyService, McpApiKeyService>();
         services.AddScoped<IPendingChangeService, PendingChangeService>();
 
+        // Register notification service (SignalR real-time notifications)
+        services.AddScoped<IMcpNotificationService, McpNotificationService>();
+
         // Register background services
         services.AddHostedService<PendingChangeExpirationBackgroundService>();