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/Shared/ColaFlow.Shared.Kernel/ColaFlow.Shared.Kernel.csproj

@@ -7,6 +7,7 @@
   </PropertyGroup>
 
   <ItemGroup>
+    <PackageReference Include="MediatR.Contracts" Version="2.0.1" />
     <PackageReference Include="Microsoft.Extensions.Configuration.Abstractions" Version="9.0.0" />
     <PackageReference Include="Microsoft.Extensions.DependencyInjection.Abstractions" Version="9.0.0" />
     <PackageReference Include="Microsoft.AspNetCore.Http.Abstractions" Version="2.2.0" />

colaflow-api/src/Shared/ColaFlow.Shared.Kernel/Events/DomainEvent.cs

@@ -1,9 +1,11 @@
+using MediatR;
+
 namespace ColaFlow.Shared.Kernel.Events;
 
 /// <summary>
 /// Base class for all domain events
 /// </summary>
-public abstract record DomainEvent
+public abstract record DomainEvent : INotification
 {
     public Guid EventId { get; init; } = Guid.NewGuid();
     public DateTime OccurredOn { get; init; } = DateTime.UtcNow;