feat(backend): Implement User Invitation System (Phase 4)

Add complete user invitation system to enable multi-user tenants.

Changes:
- Created Invitation domain entity with 7-day expiration
- Implemented InviteUserCommand with security validation
- Implemented AcceptInvitationCommand (creates user + assigns role)
- Implemented GetPendingInvitationsQuery
- Implemented CancelInvitationCommand
- Added TenantInvitationsController with tenant-scoped endpoints
- Added public invitation acceptance endpoint to AuthController
- Created database migration for invitations table
- Registered InvitationRepository in DI container
- Created domain event handlers for audit trail

Security Features:
- Cannot invite as TenantOwner or AIAgent roles
- Cross-tenant validation on all endpoints
- Secure token generation and hashing
- RequireTenantAdmin policy for invite/list
- RequireTenantOwner policy for cancel

This UNBLOCKS 3 skipped Day 6 tests (RemoveUserFromTenant).

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

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

colaflow-api/src/Modules/Identity/ColaFlow.Modules.Identity.Application/EventHandlers/InvitationAcceptedEventHandler.cs

@@ -0,0 +1,31 @@
+using ColaFlow.Modules.Identity.Domain.Aggregates.Invitations.Events;
+using MediatR;
+using Microsoft.Extensions.Logging;
+
+namespace ColaFlow.Modules.Identity.Application.EventHandlers;
+
+/// <summary>
+/// Event handler for InvitationAcceptedEvent - logs acceptance
+/// </summary>
+public class InvitationAcceptedEventHandler : INotificationHandler<InvitationAcceptedEvent>
+{
+    private readonly ILogger<InvitationAcceptedEventHandler> _logger;
+
+    public InvitationAcceptedEventHandler(ILogger<InvitationAcceptedEventHandler> logger)
+    {
+        _logger = logger;
+    }
+
+    public Task Handle(InvitationAcceptedEvent notification, CancellationToken cancellationToken)
+    {
+        _logger.LogInformation(
+            "Invitation accepted: Email={Email}, Tenant={TenantId}, Role={Role}",
+            notification.Email,
+            notification.TenantId,
+            notification.Role);
+
+        // Future: Could send welcome email, track conversion metrics, etc.
+
+        return Task.CompletedTask;
+    }
+}

colaflow-api/src/Modules/Identity/ColaFlow.Modules.Identity.Application/EventHandlers/InvitationCancelledEventHandler.cs

@@ -0,0 +1,30 @@
+using ColaFlow.Modules.Identity.Domain.Aggregates.Invitations.Events;
+using MediatR;
+using Microsoft.Extensions.Logging;
+
+namespace ColaFlow.Modules.Identity.Application.EventHandlers;
+
+/// <summary>
+/// Event handler for InvitationCancelledEvent - logs cancellation
+/// </summary>
+public class InvitationCancelledEventHandler : INotificationHandler<InvitationCancelledEvent>
+{
+    private readonly ILogger<InvitationCancelledEventHandler> _logger;
+
+    public InvitationCancelledEventHandler(ILogger<InvitationCancelledEventHandler> logger)
+    {
+        _logger = logger;
+    }
+
+    public Task Handle(InvitationCancelledEvent notification, CancellationToken cancellationToken)
+    {
+        _logger.LogInformation(
+            "Invitation cancelled: Email={Email}, Tenant={TenantId}",
+            notification.Email,
+            notification.TenantId);
+
+        // Future: Could notify invited user, track cancellation metrics, etc.
+
+        return Task.CompletedTask;
+    }
+}

colaflow-api/src/Modules/Identity/ColaFlow.Modules.Identity.Application/EventHandlers/UserInvitedEventHandler.cs

@@ -0,0 +1,32 @@
+using ColaFlow.Modules.Identity.Domain.Aggregates.Invitations.Events;
+using MediatR;
+using Microsoft.Extensions.Logging;
+
+namespace ColaFlow.Modules.Identity.Application.EventHandlers;
+
+/// <summary>
+/// Event handler for UserInvitedEvent - logs invitation
+/// </summary>
+public class UserInvitedEventHandler : INotificationHandler<UserInvitedEvent>
+{
+    private readonly ILogger<UserInvitedEventHandler> _logger;
+
+    public UserInvitedEventHandler(ILogger<UserInvitedEventHandler> logger)
+    {
+        _logger = logger;
+    }
+
+    public Task Handle(UserInvitedEvent notification, CancellationToken cancellationToken)
+    {
+        _logger.LogInformation(
+            "User invited: Email={Email}, Tenant={TenantId}, Role={Role}, InvitedBy={InvitedBy}",
+            notification.Email,
+            notification.TenantId,
+            notification.Role,
+            notification.InvitedBy);
+
+        // Future: Could add analytics tracking, audit log entry, etc.
+
+        return Task.CompletedTask;
+    }
+}