Add MassData feature with API, paging, and Blazor grid

Introduces MassData management to backend and Blazor frontend:
- Adds API endpoint for MassData count and paging
- Updates repository and controller for count support
- Implements MediatR query/handler for count
- Adds Blazor page and grid for viewing/editing MassData
- Registers MassDataApiClient and integrates with DI
- Supports paging, upsert, and UI feedback in grid

DbFirst.Application/MassData/Queries/GetMassDataCountQuery.cs

@@ -0,0 +1,5 @@
+using MediatR;
+
+namespace DbFirst.Application.MassData.Queries;
+
+public record GetMassDataCountQuery : IRequest<int>;