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/LoginWith2fa.cshtml.cs

@@ -0,0 +1,131 @@
+// 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.ComponentModel.DataAnnotations;
+using System.Threading.Tasks;
+using Microsoft.AspNetCore.Authorization;
+using Microsoft.AspNetCore.Mvc;
+using Microsoft.AspNetCore.Mvc.RazorPages;
+using Microsoft.Extensions.Logging;
+using Microsoft.AspNetCore.Identity;
+using Microsoft.Extensions.Logging;
+
+namespace turf_tasker.Areas.Identity.Pages.Account
+{
+    public class LoginWith2faModel : PageModel
+    {
+        private readonly SignInManager<IdentityUser> _signInManager;
+        private readonly UserManager<IdentityUser> _userManager;
+        private readonly ILogger<LoginWith2faModel> _logger;
+
+        public LoginWith2faModel(
+            SignInManager<IdentityUser> signInManager,
+            UserManager<IdentityUser> userManager,
+            ILogger<LoginWith2faModel> logger)
+        {
+            _signInManager = signInManager;
+            _userManager = userManager;
+            _logger = logger;
+        }
+
+        /// <summary>
+        ///     This API supports the ASP.NET Core Identity default UI infrastructure and is not intended to be used
+        ///     directly from your code. This API may change or be removed in future releases.
+        /// </summary>
+        [BindProperty]
+        public InputModel Input { get; set; }
+
+        /// <summary>
+        ///     This API supports the ASP.NET Core Identity default UI infrastructure and is not intended to be used
+        ///     directly from your code. This API may change or be removed in future releases.
+        /// </summary>
+        public bool RememberMe { get; set; }
+
+        /// <summary>
+        ///     This API supports the ASP.NET Core Identity default UI infrastructure and is not intended to be used
+        ///     directly from your code. This API may change or be removed in future releases.
+        /// </summary>
+        public string ReturnUrl { get; set; }
+
+        /// <summary>
+        ///     This API supports the ASP.NET Core Identity default UI infrastructure and is not intended to be used
+        ///     directly from your code. This API may change or be removed in future releases.
+        /// </summary>
+        public class InputModel
+        {
+            /// <summary>
+            ///     This API supports the ASP.NET Core Identity default UI infrastructure and is not intended to be used
+            ///     directly from your code. This API may change or be removed in future releases.
+            /// </summary>
+            [Required]
+            [StringLength(7, ErrorMessage = "The {0} must be at least {2} and at max {1} characters long.", MinimumLength = 6)]
+            [DataType(DataType.Text)]
+            [Display(Name = "Authenticator code")]
+            public string TwoFactorCode { get; set; }
+
+            /// <summary>
+            ///     This API supports the ASP.NET Core Identity default UI infrastructure and is not intended to be used
+            ///     directly from your code. This API may change or be removed in future releases.
+            /// </summary>
+            [Display(Name = "Remember this machine")]
+            public bool RememberMachine { get; set; }
+        }
+
+        public async Task<IActionResult> OnGetAsync(bool rememberMe, string returnUrl = null)
+        {
+            // Ensure the user has gone through the username & password screen first
+            var user = await _signInManager.GetTwoFactorAuthenticationUserAsync();
+
+            if (user == null)
+            {
+                throw new InvalidOperationException($"Unable to load two-factor authentication user.");
+            }
+
+            ReturnUrl = returnUrl;
+            RememberMe = rememberMe;
+
+            return Page();
+        }
+
+        public async Task<IActionResult> OnPostAsync(bool rememberMe, string returnUrl = null)
+        {
+            if (!ModelState.IsValid)
+            {
+                return Page();
+            }
+
+            returnUrl = returnUrl ?? Url.Content("~/");
+
+            var user = await _signInManager.GetTwoFactorAuthenticationUserAsync();
+            if (user == null)
+            {
+                throw new InvalidOperationException($"Unable to load two-factor authentication user.");
+            }
+
+            var authenticatorCode = Input.TwoFactorCode.Replace(" ", string.Empty).Replace("-", string.Empty);
+
+            var result = await _signInManager.TwoFactorAuthenticatorSignInAsync(authenticatorCode, rememberMe, Input.RememberMachine);
+
+            var userId = await _userManager.GetUserIdAsync(user);
+
+            if (result.Succeeded)
+            {
+                _logger.LogInformation("User with ID '{UserId}' logged in with 2fa.", user.Id);
+                return LocalRedirect(returnUrl);
+            }
+            else if (result.IsLockedOut)
+            {
+                _logger.LogWarning("User with ID '{UserId}' account locked out.", user.Id);
+                return RedirectToPage("./Lockout");
+            }
+            else
+            {
+                _logger.LogWarning("Invalid authenticator code entered for user with ID '{UserId}'.", user.Id);
+                ModelState.AddModelError(string.Empty, "Invalid authenticator code.");
+                return Page();
+            }
+        }
+    }
+}