refactor: rename namespace to FiscalFlow and upgrade to .NET 10

- Rename InvoiceMaster.* to FiscalFlow.* namespace
- Upgrade from .NET 8 to .NET 10
- Update all NuGet packages to latest versions
- Update C# language version to 14.0

backend/src/FiscalFlow.Core/Interfaces/IBlobStorageService.cs

@@ -0,0 +1,10 @@
+namespace FiscalFlow.Core.Interfaces;
+
+public interface IBlobStorageService
+{
+    Task<string> UploadAsync(string fileName, Stream content, string contentType, CancellationToken cancellationToken = default);
+    Task<Stream> DownloadAsync(string blobName, CancellationToken cancellationToken = default);
+    Task DeleteAsync(string blobName, CancellationToken cancellationToken = default);
+    Task<bool> ExistsAsync(string blobName, CancellationToken cancellationToken = default);
+    string GetBlobUrl(string blobName);
+}