Extract IAuditService, drop virtual mock seam from AuditService

Consumers now depend on IAuditService instead of the concrete class,
so tests mock the interface (Mock<IAuditService>()) rather than
subclassing AuditService via virtual methods.

src/api/MicCheck.Api/Projects/ProjectService.cs

@@ -5,7 +5,7 @@ using Microsoft.EntityFrameworkCore;
 
 namespace MicCheck.Api.Projects;
 
-public class ProjectService(IMicCheckDbContext db, AuditService auditService)
+public class ProjectService(IMicCheckDbContext db, IAuditService auditService)
 {
     public async Task<IReadOnlyList<Project>> ListByOrganizationAsync(int organizationId, CancellationToken ct = default)
     {