feat: Integrate ASP.NET Core Identity for user authentication

This commit integrates ASP.NET Core Identity into the application to enable user registration, login, and management. This lays the groundwork for securing data per user.

**Key Changes:**

*   **DbContext Configuration:**
    *   Modified `ApplicationDbContext.cs` to inherit from `IdentityDbContext<IdentityUser>`.
    *   Removed an unnecessary `using` statement from `ApplicationDbContext.cs`.

*   **Program.cs Setup:**
    *   Configured `AddDefaultIdentity<IdentityUser>` with `AddEntityFrameworkStores<ApplicationDbContext>()` to register Identity services.
    *   Ensured correct ordering of `UseAuthentication()` and `UseAuthorization()` middleware.
    *   Added `app.MapRazorPages()` to enable the Identity UI pages.
    *   Verified core package versions in `turf_tasker.csproj` for consistency across EF Core and Identity components (`8.0.6`).

*   **Identity UI:**
    *   Scaffolded ASP.NET Core Identity pages (Login, Register, Manage, etc.) to provide the user interface for authentication.
    *   Added a `_LoginPartial.cshtml` partial view to the `Views/Shared` folder.
    *   Rendered `_LoginPartial` in `Views/Shared/_Layout.cshtml` to display login/register/logout links in the navigation bar.

*   **Migrations:**
    *   Created and applied a new migration (`AddIdentitySchema`) to create the necessary ASP.NET Core Identity database tables (e.g., `AspNetUsers`, `AspNetRoles`).

Areas/Identity/Pages/Account/Logout.cshtml.cs

@@ -0,0 +1,42 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+#nullable disable
+
+using System;
+using System.Threading.Tasks;
+using Microsoft.AspNetCore.Authorization;
+using Microsoft.AspNetCore.Identity;
+using Microsoft.AspNetCore.Mvc;
+using Microsoft.AspNetCore.Mvc.RazorPages;
+using Microsoft.Extensions.Logging;
+
+namespace turf_tasker.Areas.Identity.Pages.Account
+{
+    public class LogoutModel : PageModel
+    {
+        private readonly SignInManager<IdentityUser> _signInManager;
+        private readonly ILogger<LogoutModel> _logger;
+
+        public LogoutModel(SignInManager<IdentityUser> signInManager, ILogger<LogoutModel> logger)
+        {
+            _signInManager = signInManager;
+            _logger = logger;
+        }
+
+        public async Task<IActionResult> OnPost(string returnUrl = null)
+        {
+            await _signInManager.SignOutAsync();
+            _logger.LogInformation("User logged out.");
+            if (returnUrl != null)
+            {
+                return LocalRedirect(returnUrl);
+            }
+            else
+            {
+                // This needs to be a redirect so that the browser performs a new
+                // request and the identity for the user gets updated.
+                return RedirectToPage();
+            }
+        }
+    }
+}