feat(backend): Implement PendingChange Management (Story 5.10)

Implemented complete Human-in-the-Loop approval workflow for AI-proposed changes:

Changes:
- Created PendingChange DTOs (PendingChangeDto, CreatePendingChangeRequest, ApproveChangeRequest, RejectChangeRequest, PendingChangeFilterDto)
- Implemented IPendingChangeService interface with CRUD, approval/rejection, expiration, and deletion operations
- Implemented PendingChangeService with full workflow support and tenant isolation
- Created McpPendingChangesController REST API with endpoints for listing, approving, rejecting, and deleting pending changes
- Implemented PendingChangeApprovedEventHandler to execute approved changes via MediatR commands (Project, Epic, Story, Task CRUD operations)
- Created PendingChangeExpirationBackgroundService for auto-expiration of changes after 24 hours
- Registered all services and background service in DI container

Technical Details:
- Status flow: PendingApproval → Approved → Applied (or Rejected/Expired)
- Tenant isolation enforced in all operations
- Domain events published for audit trail
- Event-driven execution using MediatR
- Background service runs every 5 minutes to expire old changes
- JWT authentication required for all endpoints

🤖 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/ApproveChangeRequest.cs

@@ -0,0 +1,12 @@
+namespace ColaFlow.Modules.Mcp.Application.DTOs;
+
+/// <summary>
+/// Request to approve a PendingChange
+/// </summary>
+public class ApproveChangeRequest
+{
+    // Currently empty, but we may add fields later like:
+    // - ApprovalComments
+    // - AutoApply flag
+    // - ScheduledExecutionTime
+}

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

@@ -0,0 +1,13 @@
+using ColaFlow.Modules.Mcp.Domain.ValueObjects;
+
+namespace ColaFlow.Modules.Mcp.Application.DTOs;
+
+/// <summary>
+/// Request to create a new PendingChange
+/// </summary>
+public class CreatePendingChangeRequest
+{
+    public string ToolName { get; set; } = null!;
+    public DiffPreview Diff { get; set; } = null!;
+    public int ExpirationHours { get; set; } = 24;
+}

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

@@ -0,0 +1,29 @@
+using ColaFlow.Modules.Mcp.Domain.ValueObjects;
+
+namespace ColaFlow.Modules.Mcp.Application.DTOs;
+
+/// <summary>
+/// DTO for PendingChange response
+/// </summary>
+public class PendingChangeDto
+{
+    public Guid Id { get; set; }
+    public Guid TenantId { get; set; }
+    public Guid ApiKeyId { get; set; }
+    public string ToolName { get; set; } = null!;
+    public DiffPreviewDto Diff { get; set; } = null!;
+    public string Status { get; set; } = null!;
+    public DateTime CreatedAt { get; set; }
+    public DateTime ExpiresAt { get; set; }
+    public Guid? ApprovedBy { get; set; }
+    public DateTime? ApprovedAt { get; set; }
+    public Guid? RejectedBy { get; set; }
+    public DateTime? RejectedAt { get; set; }
+    public string? RejectionReason { get; set; }
+    public DateTime? AppliedAt { get; set; }
+    public string? ApplicationResult { get; set; }
+    public bool IsExpired { get; set; }
+    public bool CanBeApproved { get; set; }
+    public bool CanBeRejected { get; set; }
+    public string Summary { get; set; } = null!;
+}

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

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+using ColaFlow.Modules.Mcp.Domain.ValueObjects;
+
+namespace ColaFlow.Modules.Mcp.Application.DTOs;
+
+/// <summary>
+/// Filter options for querying PendingChanges
+/// </summary>
+public class PendingChangeFilterDto
+{
+    public PendingChangeStatus? Status { get; set; }
+    public string? EntityType { get; set; }
+    public Guid? EntityId { get; set; }
+    public Guid? ApiKeyId { get; set; }
+    public string? ToolName { get; set; }
+    public bool? IncludeExpired { get; set; }
+    public int Page { get; set; } = 1;
+    public int PageSize { get; set; } = 20;
+}

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

@@ -0,0 +1,9 @@
+namespace ColaFlow.Modules.Mcp.Application.DTOs;
+
+/// <summary>
+/// Request to reject a PendingChange
+/// </summary>
+public class RejectChangeRequest
+{
+    public string Reason { get; set; } = null!;
+}