feat(backend): Implement Domain Events infrastructure and handlers

Add complete domain events dispatching infrastructure and critical event handlers for Identity module.

Changes:
- Added IMediator injection to IdentityDbContext
- Implemented SaveChangesAsync override to dispatch domain events before persisting
- Made DomainEvent base class implement INotification (added MediatR.Contracts dependency)
- Created 3 new domain events: UserRoleAssignedEvent, UserRemovedFromTenantEvent, UserLoggedInEvent
- Implemented 4 event handlers with structured logging:
  - UserRoleAssignedEventHandler (audit log, cache invalidation placeholder)
  - UserRemovedFromTenantEventHandler (notification placeholder)
  - UserLoggedInEventHandler (login tracking placeholder)
  - TenantCreatedEventHandler (welcome email placeholder)
- Updated unit tests to inject mock IMediator into IdentityDbContext

Technical Details:
- Domain events are now published via MediatR within the same transaction
- Events are dispatched BEFORE SaveChangesAsync to ensure atomicity
- Event handlers auto-registered by MediatR assembly scanning
- All handlers include structured logging for observability

Next Steps (Phase 3):
- Update command handlers to raise new events (UserLoggedInEvent, UserRoleAssignedEvent)
- Add event raising logic to User/Tenant aggregates
- Implement audit logging persistence (currently just logging)

🤖 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/TenantCreatedEventHandler.cs

@@ -0,0 +1,29 @@
+using ColaFlow.Modules.Identity.Domain.Aggregates.Tenants.Events;
+using MediatR;
+using Microsoft.Extensions.Logging;
+
+namespace ColaFlow.Modules.Identity.Application.EventHandlers;
+
+public sealed class TenantCreatedEventHandler : INotificationHandler<TenantCreatedEvent>
+{
+    private readonly ILogger<TenantCreatedEventHandler> _logger;
+
+    public TenantCreatedEventHandler(ILogger<TenantCreatedEventHandler> logger)
+    {
+        _logger = logger;
+    }
+
+    public Task Handle(TenantCreatedEvent notification, CancellationToken cancellationToken)
+    {
+        _logger.LogInformation(
+            "Tenant {TenantId} created with slug '{Slug}'",
+            notification.TenantId,
+            notification.Slug);
+
+        // Future: Send welcome email
+        // Future: Initialize default settings
+        // Future: Create audit log entry
+
+        return Task.CompletedTask;
+    }
+}

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

@@ -0,0 +1,30 @@
+using ColaFlow.Modules.Identity.Domain.Aggregates.Users.Events;
+using MediatR;
+using Microsoft.Extensions.Logging;
+
+namespace ColaFlow.Modules.Identity.Application.EventHandlers;
+
+public sealed class UserLoggedInEventHandler : INotificationHandler<UserLoggedInEvent>
+{
+    private readonly ILogger<UserLoggedInEventHandler> _logger;
+
+    public UserLoggedInEventHandler(ILogger<UserLoggedInEventHandler> logger)
+    {
+        _logger = logger;
+    }
+
+    public Task Handle(UserLoggedInEvent notification, CancellationToken cancellationToken)
+    {
+        _logger.LogInformation(
+            "User {UserId} logged in to tenant {TenantId} from IP {IpAddress}",
+            notification.UserId,
+            notification.TenantId.Value,
+            notification.IpAddress);
+
+        // Future: Track login history
+        // Future: Detect suspicious login patterns
+        // Future: Update last login timestamp
+
+        return Task.CompletedTask;
+    }
+}

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

@@ -0,0 +1,30 @@
+using ColaFlow.Modules.Identity.Domain.Aggregates.Users.Events;
+using MediatR;
+using Microsoft.Extensions.Logging;
+
+namespace ColaFlow.Modules.Identity.Application.EventHandlers;
+
+public sealed class UserRemovedFromTenantEventHandler : INotificationHandler<UserRemovedFromTenantEvent>
+{
+    private readonly ILogger<UserRemovedFromTenantEventHandler> _logger;
+
+    public UserRemovedFromTenantEventHandler(ILogger<UserRemovedFromTenantEventHandler> logger)
+    {
+        _logger = logger;
+    }
+
+    public Task Handle(UserRemovedFromTenantEvent notification, CancellationToken cancellationToken)
+    {
+        _logger.LogInformation(
+            "User {UserId} removed from tenant {TenantId}. Removed by: {RemovedBy}. Reason: {Reason}",
+            notification.UserId,
+            notification.TenantId.Value,
+            notification.RemovedBy,
+            notification.Reason);
+
+        // Future: Send notification to user
+        // Future: Audit log
+
+        return Task.CompletedTask;
+    }
+}

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

@@ -0,0 +1,32 @@
+using ColaFlow.Modules.Identity.Domain.Aggregates.Users.Events;
+using MediatR;
+using Microsoft.Extensions.Logging;
+
+namespace ColaFlow.Modules.Identity.Application.EventHandlers;
+
+public sealed class UserRoleAssignedEventHandler : INotificationHandler<UserRoleAssignedEvent>
+{
+    private readonly ILogger<UserRoleAssignedEventHandler> _logger;
+
+    public UserRoleAssignedEventHandler(ILogger<UserRoleAssignedEventHandler> logger)
+    {
+        _logger = logger;
+    }
+
+    public Task Handle(UserRoleAssignedEvent notification, CancellationToken cancellationToken)
+    {
+        _logger.LogInformation(
+            "User {UserId} assigned role {Role} in tenant {TenantId}. Previous role: {PreviousRole}. Assigned by: {AssignedBy}",
+            notification.UserId,
+            notification.Role,
+            notification.TenantId.Value,
+            notification.PreviousRole,
+            notification.AssignedBy);
+
+        // Future: Add audit log to database
+        // Future: Invalidate user permissions cache
+        // Future: Send notification to user
+
+        return Task.CompletedTask;
+    }
+}