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/Middleware/ExceptionHandlingMiddleware.cs

@@ -0,0 +1,77 @@
+using Microsoft.AspNetCore.Mvc;
+using System.Net;
+
+namespace FiscalFlow.API.Middleware;
+
+public class ExceptionHandlingMiddleware
+{
+    private readonly RequestDelegate _next;
+    private readonly ILogger<ExceptionHandlingMiddleware> _logger;
+
+    public ExceptionHandlingMiddleware(RequestDelegate next, ILogger<ExceptionHandlingMiddleware> logger)
+    {
+        _next = next;
+        _logger = logger;
+    }
+
+    public async Task InvokeAsync(HttpContext context)
+    {
+        try
+        {
+            await _next(context);
+        }
+        catch (Exception ex)
+        {
+            _logger.LogError(ex, "An unhandled exception occurred");
+            await HandleExceptionAsync(context, ex);
+        }
+    }
+
+    private static Task HandleExceptionAsync(HttpContext context, Exception exception)
+    {
+        context.Response.ContentType = "application/json";
+        
+        var (statusCode, errorCode, message) = exception switch
+        {
+            UnauthorizedAccessException _ => (
+                (int)HttpStatusCode.Unauthorized, 
+                "UNAUTHORIZED", 
+                "Authentication required"
+            ),
+            InvalidOperationException _ => (
+                (int)HttpStatusCode.BadRequest, 
+                "INVALID_OPERATION", 
+                exception.Message
+            ),
+            KeyNotFoundException _ => (
+                (int)HttpStatusCode.NotFound, 
+                "NOT_FOUND", 
+                "Resource not found"
+            ),
+            _ => (
+                (int)HttpStatusCode.InternalServerError, 
+                "INTERNAL_ERROR", 
+                "An unexpected error occurred"
+            )
+        };
+
+        context.Response.StatusCode = statusCode;
+
+        var response = new
+        {
+            success = false,
+            error = new
+            {
+                code = errorCode,
+                message
+            },
+            meta = new
+            {
+                request_id = context.TraceIdentifier,
+                timestamp = DateTime.UtcNow.ToString("O")
+            }
+        };
+
+        return context.Response.WriteAsJsonAsync(response);
+    }
+}