backend phase 0: foundation, cross-cutting seams & starter cleanup

Remove the Order demo (entity/feature/repo/config/gRPC/proto) and the three
pre-marketplace migrations; regenerate a fresh InitialBaseline migration.

Stand up the REST surface (PingController + System/Ping CQRS) proving the
Mediator -> behaviors -> OperationResult -> ApiResult envelope end to end.

Close wiring gaps: register LoggingBehavior (outermost) and add the built-in
rate limiter (per-IP global + otp/auth/sensitive policies), placed before
authentication.

Add current-user + audit plumbing: ICurrentUser (HttpContext + null impls),
rename BaseEntity audit fields to CreatedAt/ModifiedAt (DateTimeOffset) +
CreatedById/ModifiedById, stamped by a new AuditFieldInterceptor.

Introduce five cross-cutting seams (IDateTimeProvider, IFieldEncryptor,
ICacheService, IObjectStorage, INotificationDispatcher) with in-memory/local
mocks registered via AddCrossCuttingSeams.

Add Baya.Test.Foundation (encryptor, audit interceptor, ping handler) and
update docs, contracts (swagger.v1.json), handoff, report, and mocks registry.

server/src/Core/Baya.Application/Baya.Application.csproj

@@ -10,6 +10,9 @@
     <EmbeddedResource Remove="Common\ValidationBase\**" />
     <None Remove="Common\ValidationBase\**" />
   </ItemGroup>
+  <ItemGroup>
+    <InternalsVisibleTo Include="Baya.Test.Foundation" />
+  </ItemGroup>
   <ItemGroup>
     <ProjectReference Include="..\Baya.Domain\Baya.Domain.csproj" />
     <PackageReference Include="Mediator.SourceGenerator">