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/API/Plugins/Baya.Web.Plugins.Grpc/GrpcPluginStartup.cs

@@ -21,7 +21,6 @@ public static class GrpcPluginStartup
     {
 
         app.MapGrpcService<UserGrpcServices>();
-        app.MapGrpcService<OrderGrpcServices>();
         app.MapGrpcReflectionService();
 
         app.MapGet("/GrpcUser", async context =>
@@ -29,11 +28,5 @@ public static class GrpcPluginStartup
             await context.Response.WriteAsync(
                 "Communication with this gRPC endpoint must be made through a gRPC client.");
         });
-
-        app.MapGet("/GrpcUserOrder", async context =>
-        {
-            await context.Response.WriteAsync(
-                "Communication with this gRPC endpoint must be made through a gRPC client.");
-        });
     }
 }