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.Api/Controllers/InvoicesController.cs

@@ -0,0 +1,67 @@
+using FiscalFlow.Application.Commands.Invoices;
+using MediatR;
+using Microsoft.AspNetCore.Authorization;
+using Microsoft.AspNetCore.Mvc;
+using System.Security.Claims;
+
+namespace FiscalFlow.API.Controllers;
+
+[ApiController]
+[Route("api/v1/invoices")]
+[Authorize]
+public class InvoicesController : ControllerBase
+{
+    private readonly IMediator _mediator;
+
+    public InvoicesController(IMediator mediator)
+    {
+        _mediator = mediator;
+    }
+
+    [HttpPost]
+    [Consumes("multipart/form-data")]
+    public async Task<IActionResult> UploadInvoice([FromForm] UploadInvoiceRequest request)
+    {
+        var userId = GetUserId();
+        
+        if (request.File == null || request.File.Length == 0)
+        {
+            return BadRequest(new { success = false, error = "No file uploaded" });
+        }
+
+        if (!request.File.ContentType.Equals("application/pdf", StringComparison.OrdinalIgnoreCase))
+        {
+            return BadRequest(new { success = false, error = "Only PDF files are supported" });
+        }
+
+        var command = new UploadInvoiceCommand(
+            userId,
+            request.Provider,
+            request.File.FileName,
+            request.File.OpenReadStream(),
+            request.File.Length,
+            request.File.ContentType);
+
+        var result = await _mediator.Send(command);
+
+        if (!result.Success)
+        {
+            return BadRequest(new { success = false, error = result.ErrorMessage });
+        }
+
+        return Ok(new { success = true, data = result.Invoice });
+    }
+
+    private Guid GetUserId()
+    {
+        var userId = User.FindFirst(ClaimTypes.NameIdentifier)?.Value;
+        return Guid.Parse(userId!);
+    }
+}
+
+public class UploadInvoiceRequest
+{
+    public IFormFile? File { get; set; }
+    public string Provider { get; set; } = "fortnox";
+    public bool AutoProcess { get; set; } = false;
+}