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.Application/DTOs/Notifications/PendingChangeAppliedNotification.cs

@@ -0,0 +1,18 @@
+namespace ColaFlow.Modules.Mcp.Application.DTOs.Notifications;
+
+/// <summary>
+/// Notification sent when a PendingChange has been successfully applied
+/// (after approval and execution)
+/// </summary>
+public sealed record PendingChangeAppliedNotification : PendingChangeNotification
+{
+    /// <summary>
+    /// Result of applying the change
+    /// </summary>
+    public required string Result { get; init; }
+
+    /// <summary>
+    /// When the change was applied (UTC)
+    /// </summary>
+    public required DateTime AppliedAt { get; init; }
+}

colaflow-api/src/Modules/Mcp/ColaFlow.Modules.Mcp.Application/DTOs/Notifications/PendingChangeApprovedNotification.cs

@@ -0,0 +1,32 @@
+namespace ColaFlow.Modules.Mcp.Application.DTOs.Notifications;
+
+/// <summary>
+/// Notification sent when a PendingChange is approved and executed
+/// </summary>
+public sealed record PendingChangeApprovedNotification : PendingChangeNotification
+{
+    /// <summary>
+    /// Type of entity that was changed
+    /// </summary>
+    public required string EntityType { get; init; }
+
+    /// <summary>
+    /// Operation that was performed
+    /// </summary>
+    public required string Operation { get; init; }
+
+    /// <summary>
+    /// ID of the entity that was created/updated (if applicable)
+    /// </summary>
+    public Guid? EntityId { get; init; }
+
+    /// <summary>
+    /// ID of the user who approved the change
+    /// </summary>
+    public required Guid ApprovedBy { get; init; }
+
+    /// <summary>
+    /// Result of executing the change (e.g., "Epic created: {id} - {name}")
+    /// </summary>
+    public string? ExecutionResult { get; init; }
+}

colaflow-api/src/Modules/Mcp/ColaFlow.Modules.Mcp.Application/DTOs/Notifications/PendingChangeCreatedNotification.cs

@@ -0,0 +1,22 @@
+namespace ColaFlow.Modules.Mcp.Application.DTOs.Notifications;
+
+/// <summary>
+/// Notification sent when a new PendingChange is created
+/// </summary>
+public sealed record PendingChangeCreatedNotification : PendingChangeNotification
+{
+    /// <summary>
+    /// Type of entity being changed (Epic, Story, Task, etc.)
+    /// </summary>
+    public required string EntityType { get; init; }
+
+    /// <summary>
+    /// Operation being performed (CREATE, UPDATE, DELETE)
+    /// </summary>
+    public required string Operation { get; init; }
+
+    /// <summary>
+    /// Summary of what will be changed
+    /// </summary>
+    public required string Summary { get; init; }
+}

colaflow-api/src/Modules/Mcp/ColaFlow.Modules.Mcp.Application/DTOs/Notifications/PendingChangeExpiredNotification.cs

@@ -0,0 +1,12 @@
+namespace ColaFlow.Modules.Mcp.Application.DTOs.Notifications;
+
+/// <summary>
+/// Notification sent when a PendingChange expires (timeout)
+/// </summary>
+public sealed record PendingChangeExpiredNotification : PendingChangeNotification
+{
+    /// <summary>
+    /// When the pending change expired (UTC)
+    /// </summary>
+    public required DateTime ExpiredAt { get; init; }
+}

colaflow-api/src/Modules/Mcp/ColaFlow.Modules.Mcp.Application/DTOs/Notifications/PendingChangeNotification.cs

@@ -0,0 +1,32 @@
+namespace ColaFlow.Modules.Mcp.Application.DTOs.Notifications;
+
+/// <summary>
+/// Base class for all PendingChange notifications
+/// </summary>
+public abstract record PendingChangeNotification
+{
+    /// <summary>
+    /// Type of notification (PendingChangeCreated, PendingChangeApproved, etc.)
+    /// </summary>
+    public required string NotificationType { get; init; }
+
+    /// <summary>
+    /// The ID of the pending change
+    /// </summary>
+    public required Guid PendingChangeId { get; init; }
+
+    /// <summary>
+    /// The tool that created the pending change
+    /// </summary>
+    public required string ToolName { get; init; }
+
+    /// <summary>
+    /// When this notification was generated (UTC)
+    /// </summary>
+    public DateTime Timestamp { get; init; } = DateTime.UtcNow;
+
+    /// <summary>
+    /// Tenant ID for multi-tenancy support
+    /// </summary>
+    public required Guid TenantId { get; init; }
+}

colaflow-api/src/Modules/Mcp/ColaFlow.Modules.Mcp.Application/DTOs/Notifications/PendingChangeRejectedNotification.cs

@@ -0,0 +1,17 @@
+namespace ColaFlow.Modules.Mcp.Application.DTOs.Notifications;
+
+/// <summary>
+/// Notification sent when a PendingChange is rejected
+/// </summary>
+public sealed record PendingChangeRejectedNotification : PendingChangeNotification
+{
+    /// <summary>
+    /// Reason for rejection
+    /// </summary>
+    public required string Reason { get; init; }
+
+    /// <summary>
+    /// ID of the user who rejected the change
+    /// </summary>
+    public required Guid RejectedBy { get; init; }
+}